Integrating Mac OS X and clang compiler into Travis CI (#601)

changes: 
- .travis.yml:
  - added matrix for different osx's, since enumeration style only works for `env` and `compiler`

- scripts/travis/build.sh:
  - changed getting compiler version from ``${CXX} -dumpversion`` to ``${CXX} --version | grep -m1 ""`` 
  - added `--oversubscribe` flag to `mpiexec` on Mac to allow more processes on a node than processing elements
  - added `--mca btl tcp,self` flag for Open MPI to use the "tcp" and "self" BTLs for transporting MPI messages on Mac

.travis.yml

+######## Testing minimal compiler requirements ########
+# GCC          6.4.0
+# Clang        4.0
+# Apple Clang  900.0.39.2
+#######################################################
+
 language: cpp
 sudo: false
-os: linux
-dist: trusty
-compiler: gcc
 
-addons:
-    apt:
+matrix:
+  include:
+########################## OS X #########################
+## test gcc6 - single node/rank with threading backend ##
+  - name: "osx, gcc, serial"
+    os: osx
+    osx_image: xcode9.2
+    env:
+      - MATRIX_EVAL="brew install gcc@6 && brew link --force --overwrite gcc@6 && CC=gcc-6 && CXX=g++-6"
+      - BUILD_NAME=cthread WITH_DISTRIBUTED=serial
+    compiler: gcc-6
+
+## test gcc6 - mpi with threading backend ##
+  - name: "osx, gcc, mpi"
+    os: osx
+    osx_image: xcode9.2
+    env:
+      - MATRIX_EVAL="brew install gcc@6 && brew link --force --overwrite gcc@6 && CC=gcc-6 && CXX=g++-6"
+      - BUILD_NAME=mpi WITH_DISTRIBUTED=mpi
+    compiler: gcc-6
+
+## test clang9 - single node/rank with threading backend ##
+  - name: "osx, apple clang, serial"
+    os: osx
+    osx_image: xcode9.2
+    env:
+      - MATRIX_EVAL="CC=clang && CXX=clang++"
+      - BUILD_NAME=cthread WITH_DISTRIBUTED=serial
+    compiler: clang
+
+## test clang9 - mpi with threading backend ##
+  - name: "osx, apple clang, mpi"
+    os: osx
+    osx_image: xcode9.2
+    env:
+      - MATRIX_EVAL="CC=clang && CXX=clang++"
+      - BUILD_NAME=mpi WITH_DISTRIBUTED=mpi
+    compiler: clang
+
+######################### LINUX #########################
+## test gcc6 - single node/rank with threading backend ##
+  - name: "linux, gcc, serial"
+    os: linux
+    dist: trusty
+    addons:
+      apt:
         sources:
-            - ubuntu-toolchain-r-test
+          - ubuntu-toolchain-r-test
         packages:
-            - g++-6
-            - openmpi-bin
-            - libopenmpi-dev
+          - g++-6
+          - openmpi-bin
+          - libopenmpi-dev
+    env:
+      - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
+      - BUILD_NAME=cthread WITH_DISTRIBUTED=serial
+    compiler: gcc-6
 
-env:
-    # test single node/rank with threading backend
-    - BUILD_NAME=cthread  WITH_DISTRIBUTED=serial
-    # test mpi with threading backend
-    - BUILD_NAME=mpi      WITH_DISTRIBUTED=mpi
+## test gcc6 - mpi with threading backend ##
+  - name: "linux, gcc, mpi"
+    os: linux
+    dist: trusty
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
+        packages:
+          - g++-6
+          - openmpi-bin
+          - libopenmpi-dev
+    env:
+      - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
+      - BUILD_NAME=mpi WITH_DISTRIBUTED=mpi
+    compiler: gcc-6
+
+## test clang4 - single node/rank with threading backend ##
+  - name: "linux, clang, serial"
+    os: linux
+    dist: trusty
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
+          - llvm-toolchain-trusty-4.0
+        packages:
+          - clang-4.0
+          - g++-6
+          - openmpi-bin
+          - libopenmpi-dev
+    env:
+      - MATRIX_EVAL="CC=clang-4.0 && CXX=clang++-4.0"
+      - BUILD_NAME=cthread WITH_DISTRIBUTED=serial
+    compiler: clang-4.0
+
+## test clang4 - mpi with threading backend ##
+  - name: "linux, clang, mpi"
+    os: linux
+    dist: trusty
+    addons:
+      apt:
+        sources:
+          - ubuntu-toolchain-r-test
+          - llvm-toolchain-trusty-4.0
+        packages:
+          - clang-4.0
+          - g++-6
+          - openmpi-bin
+          - libopenmpi-dev
+    env:
+      - MATRIX_EVAL="CC=clang-4.0 && CXX=clang++-4.0"
+      - BUILD_NAME=mpi WITH_DISTRIBUTED=mpi
+    compiler: clang-4.0
 
-before_install:
-    - CC=gcc-6
-    - CXX=g++-6
+before_script:
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then export HOMEBREW_NO_AUTO_UPDATE=1; brew cask uninstall --force oclint; fi
+  - if [[ ( "$TRAVIS_OS_NAME" == "osx" ) && ( "$WITH_DISTRIBUTED" == "mpi" ) ]]; then brew install open-mpi; fi
+  - eval "${MATRIX_EVAL}"
 
 script: source ./scripts/travis/build.sh
 
+notifications:
+  email:
+    on_success: never
+    on_failure: always

doc/install.rst

@@ -44,8 +44,8 @@ We recommend using GCC or Clang, for which Arbor has been tested and optimised.
     Compiler    Min version  Notes
     =========== ============ ============================================
     GCC         6.1.0
-    Clang       4.0          Clang 3.8 and later probably work.
-    Apple Clang 9
+    Clang       4.0          Needs GCC 6 or later for standard library. 
+    Apple Clang 9            Apple LLVM version 9.0.0 (clang-900.0.39.2)
     Intel       17.0.1       Needs GCC 5 or later for standard library.
     =========== ============ ============================================
 
@@ -56,7 +56,7 @@ We recommend using GCC or Clang, for which Arbor has been tested and optimised.
     CMake should use. If these are not set, CMake will attempt to automatically choose a compiler,
     which may be too old to compile Arbor.
     For example, the default compiler chosen below by CMake was GCC 4.8.5 at ``/usr/bin/c++``,
-    so the ``CC`` and ``CXX`` variables were used to specify GCC 5.2.0 before calling ``cmake``.
+    so the ``CC`` and ``CXX`` variables were used to specify GCC 6.1.0 before calling ``cmake``.
 
     .. code-block:: bash
 
@@ -66,7 +66,7 @@ We recommend using GCC or Clang, for which Arbor has been tested and optimised.
 
         # check which version of GCC is available
         $ g++ --version
-        g++ (GCC) 5.2.0
+        g++ (GCC) 6.1.0
         Copyright (C) 2015 Free Software Foundation, Inc.
 
         # set environment variables for compilers
@@ -75,10 +75,10 @@ We recommend using GCC or Clang, for which Arbor has been tested and optimised.
         # launch CMake
         # the compiler version and path is given in the CMake output
         $ cmake ..
-        -- The C compiler identification is GNU 5.2.0
-        -- The CXX compiler identification is GNU 5.2.0
-        -- Check for working C compiler: /cm/local/apps/gcc/5.2.0/bin/gcc
-        -- Check for working C compiler: /cm/local/apps/gcc/5.2.0/bin/gcc -- works
+        -- The C compiler identification is GNU 6.1.0
+        -- The CXX compiler identification is GNU 6.1.0
+        -- Check for working C compiler: /cm/local/apps/gcc/6.1.0/bin/gcc
+        -- Check for working C compiler: /cm/local/apps/gcc/6.1.0/bin/gcc -- works
         ...
 
 .. Note::

scripts/travis/build.sh

@@ -13,10 +13,10 @@ build_path=build-${BUILD_NAME}
 #
 progress "build environment"
 
-compiler_version=`${CXX} -dumpversion`
+compiler_version=`${CXX} --version | grep -m1 ""`
 cmake_version=`cmake --version | grep version | awk '{print $3}'`
 
-echo "compiler   : ${CXX} ${compiler_version}"
+echo "compiler   : ${compiler_version}"
 echo "cmake      : ${cmake_version}"
 echo "build path : ${build_path}"
 echo "base path  : ${base_path}"
@@ -28,6 +28,14 @@ if [[ "${WITH_DISTRIBUTED}" = "mpi" ]]; then
     CC="mpicc"
     CXX="mpicxx"
     launch="mpiexec -n 4"
+    # on mac:
+    # --oversubscribe flag allows more processes on a node than processing elements
+    # --mca btl tcp,self for Open MPI to use the "tcp" and "self" Byte Transfer Layers for transporting MPI messages
+    # "self" to deliver messages to the same rank as the sender
+    # "tcp" sends messages across TCP-based networks (Transmission Control Protocol with Internet Protocol)
+    if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
+        launch="${launch} --oversubscribe --mca btl tcp,self"
+    fi
     WITH_MPI="ON"
 else
     echo "mpi        : off"