This should fix problems with finding system-installed headers instead of the
spack-provided ones (e.g. googletest). (In some cases sites might inject
include paths via compiler settings (or other means)).

We allow for newer py-pybind11 versions than upstream; upstream pins a specific
version, but, e.g., arbor needs a newer one. Currently, this should just work
fine :).

We encountered build fails on some HPC sites due to openssl mismatches
between bare-metal tools and spack — especially curl and similar tools
for fetching.
Our build tool, waf, needs to find spack-provided libraries, e.g.
libclang.so, at runtime… so we will only set the environment variable
for this.

This one includes a fix in hxtorch that wasn't caught when testing
locally (at UHEI) against newer pybind11 versions.

This reverts merge request !199

This reverts merge request !198

This reverts merge request !195

This reverts commit 383726f9

This is a workaround for pysctrltp not detecting newer versions
of pybind11 correctly, and for `pyhxcomm` building a test binary linking
against stuff which requires linkage against libpython.
The real fix requires CR within the software stack and will only be
available later.

In spack 0.18 boost default libraries changed, see [0].

[0] https://github.com/spack/spack/blob/v0.18.1/var/spack/repos/builtin/packages/boost/package.py#L77

This reverts merge request !189

Update packages/py-pybind11/package.py, packages/fmt/fmt-attributes-cpp11_4.1.0.patch, packages/fmt/fmt-no-export-cpp11flag_3.0.0.patch, packages/fmt/fmt-use-cmake-cxx-standard_3.0.0.patch, packages/fmt/package.py

Needed for an upcoming complex neuron demo on BSS-2.