Fix build for darwin

Fix build for darwin

Fix build for darwin, when ./configured --disable-driglx-direct

- darwin ld doesn't support -Bsymbolic or --version-script, so check if ld
supports those options before using them
- define GLX_ALIAS_UNSUPPORTED as config/darwin used to, as aliasing of non-weak
symbols isn't supported
- default to -with-dri-drivers=swrast

v2:
Use -Wl,-Bsymbolic, as before, not -Bsymbolic
Test that ld --version-script works, rather than just looking for it in ld --help
Don't use -Wl,--no-undefined on darwin, either

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>