gmacros.h: Support deprecation macros better on clang-cl

Use the GCC-style definition for the deprecation warning macros so that
builds using those won't break due to how they are placed as clang-cl
seems to not support __declspec(deprecated) very well.  Also make sure
that we do indeed support the temparary disabling of deprecation
warnings on clang-cl, as the MSVC ones don't really work on clang-cl.
12 jobs for gmacros-h-support-clang-cl in 23 minutes and 20 seconds (queued for 2 seconds)