CMake fixes (#137)

* Fix CMakeLists to handle build as a subproject

When several CMake generated projects are build together, it is common
practice to have a 'superproject' CMakeLists that uses
  add_subdir(proj1)
  add_subdir(proj2)
  ...
where each subproject is a self contained CMake based project
(Example proj1=HPX, proj2=nestmc, proj3=another, ...)

CMAKE_SOURCE_DIR always points to the top level directory which
is the superproject dir in this case, whereas PROJECT_SOURCE_DIR
always points to the root of the current project() in the CMakeLists
so one shouod use PROJECT_SOURCE_DIR as this gets the relative paths
correct.

* Add option to turn off auto generation from *.mod files

* Fix #134 : Change CMake WITH_OPTION to NMC_WITH_OPTION, compiler #define to NMC_HAVE_OPTION

1) The user may select an option by saying NMC_WITH_XXX

2) This may trigger CMake to use find_package(...) or setup some
other variables. CMake can then set variable NMC_HAVE_XXX and add a
what has actually been used.

3) Code should use #ifdef NMC_HAVE_XXX to check for a feature

Old CMake/define      New CMake                 Compiler #define
----------------      ---------                 ----------------
THREADING_MODEL       NMC_THREAD_MODEL
    WITH_TBB          NMC_WITH_TBB              NMC_HAVE_TBB
    WITH_OMP          NMC_WITH_OMP              NMC_HAVE_OMP
    WITH_SERIAL       NMC_WITH_SERiAL           NMC_HAVE_SERIAL

WITH_MPI              NMC_WITH_MPI              NMC_HAVE_MPI
WITH_CUDA             NMC_WITH_CUDA             NMC_HAVE_CUDA
WITH_GPU                                        NMC_HAVE_GPU
WITH_ASSERTIONS       NMC_WITH_ASSERTIONS       NMC_HAVE_ASSERTIONS
WITH_TRACE            NMC_WITH_TRACE            NMC_HAVE_TRACE
WITH_PROFILING        NMC_WITH_PROFILING        NMC_HAVE_PROFILING

Other user visible CMake vars
-----------------------------
VECTORIZE_TARGET            -> NMC_VECTORIZE_TARGET
USE_OPTIIZED_KERNELS        -> NMC_USE_OPTIIZED_KERNELS
BUILD_VALIDATION_DATA       -> NMC_BUILD_VALIDATION_DATA
BUILD_JULIA_VALIDATION_DATA -> NMC_BUILD_JULIA_VALIDATION_DATA
BUILD_NRN_VALIDATION_DATA   -> NMC_BUILD_NRN_VALIDATION_DATA
VALIDATION_DATA_DIR         -> NMC_VALIDATION_DATA_DIR

Variables such as NMC_THREADING_MODEL and NMC_VECTORIZE_TARGET now use
enumerated cmake values so you can toggle between them in ccmake gui.
SYSTEM_TYPE_CRAY/BGQ        -> NMC_SYSTEM_TYPE (Generic/Cray/BGQ)

* Use generator expression for modcc path

Some IDE's (like Xcode for example), override the CMake binary paths
and add /Debug or /Release etc so rules that have hard coded paths
to binaries will fail.