The current definitions were mostly useful to glibc and followed its
feature_test_macros(3) documentation.

However, this means other platforms still had problems when building with
strict compilation flags. _BSD_SOURCE, for example, is only recognized by
glibc, and other platforms sometimes need _XOPEN_SOURCE instead, or even the
removal of some definitions (such as the outdate _POSIX_SOURCE one).

_POSIX_SOURCE also had to be conditionally defined in some places, as what
it enables or disables during compilation varies across systems.