C++11 is now required, so there's no point in checking for pre-C++11
versions of GCC anymore. Note that Clang defines __GNUC__ too, but
always reports itself as GCC-4.2.1-compatible (and, in practice, the
earliest C++11-compatible versions of Clang will also be have most
GCC 4.8 features).
Apple's desktop operating system was formerly called "Mac OS X" and "OS X", but since 2016 it has been called "macOS" (starting with version 10.12).
Changing across all comments and documentation to use this current terminology, except in cases where the historical versions are explicitly referenced. No code changes are made; we should consider changing those in future PRs.
Adding @defgroup and @ingroup doxygen tags into all headers
in the common folder that contain doxygen blocks.
This improves the structure, readability, and findability
of information in the resulting output.
This commit targets purely structure and does not deal with
the content of the currently existing doxygen documentation.
