convert __APPLE__ and __ULIBC__ to HAVE_LIBC_VERSIONED_SYMBOLS

convert __APPLE__ and __ULIBC__ to HAVE_LIBC_VERSIONED_SYMBOLS

In fact only gnu-libc fully supports symbol versioning, so it is
better to have a generic macro for it. This also allows to manually
disable symbol version and allows to run tests with that
configuration on gnu-libc. That testing will still not catch compat
issues, but least ensures the code can compile.

Testing for __APPLE__ and __ULIBC__ is now done by meson. More of such
checks can be added by people using other libcs.