Add explicit defaults where missing for clarity.
Support passing in NETGEN_GIT_VERSION for use when building, eg, RPM
packages using rpkg, which requires building from an untagged source
bundle, that is, not directly from a git checkout.
Add option to choose build against internal or external Pybind11; add
cmake module to find and configure against system Pybind.
re-add versioning of shared library files.
Add section for GCC-specific options setting (right now, to turn off
harmless warnings).
Warn if pybind11_stubgen is not found.
Adjust stubgen paths to work when build is not in-place in the source tree.
