Tests: try to add a test building a python module with pybind11

[GPMueller]

This is a nontrivial problem and if our build system can manage it easily somehow it will be a good proof of concept.

[GPMueller]

This is now being tried on feature-pybind-test (b3565c0).

[GPMueller]

The pybind11 test was removed again on PR #47 (same branch), because we could not figure out a missing strdup symbol.

[GPMueller]

In order to get pybind11 to work properly within clang-build, we need to specify the following configuration parameters to targets:

"target_type":   "shared library",
"output_name":   "pylib",
"output_prefix": "",
"output_suffix": py_library_extension,
"include_directories": [py_include_dir],
"flags": {
    "link": [f'-L {py_library_dir}', f'-l{py_library_name}']
},

i.e. we need to add support for output_prefix and output_suffix.
Setting these parameters will then output something like pylib.cpython-37m-darwin.so, which can be imported with a simple import pylib. In this case, py_library_extension = sysconfig.get_config_var('EXT_SUFFIX') etc.

Since clang-build runs on Python, these variables are known intrinsically and we could add parameter blocks to our platform.py to make this sort of thing easier for users. E.g.:

PLATFORM_PYTHON_INCLUDE_PATH     = _Path(_get_paths()['include'])
PLATFORM_PYTHON_LIBRARY_PATH     = _Path(_get_paths()['data']) / "libs"
PLATFORM_PYTHON_LIBRARY_NAME     = f"python{_version_info.major}{_version_info.minor}"
PLATFORM_PYTHON_EXTENSION_SUFFIX = _get_config_var('EXT_SUFFIX')