Investigate behavior of `__GNUC__` macro

[czarny247]

Assumptions:

• It's not clear if __GNUC__ is defined if you disable GNU extensions even if your compiler supports it. maybe we should open an issue to figure out what we really ought to be doing for these checks.

• It is not defined for Clang if it is set up for MSVC (Windows)

Originally posted by @bradallred in https://github.com/gemrb/gemrb/pull/1969#discussion_r1433162057

[lynxlynxlynx]

If we want to avoid using it, it's as simple as adding 5 more cmake tests for compiler features. Namely:

• __attribute__ ((deprecated
• __attribute__ ((visibility
• __attribute__ ((warn_unused_result))
• __attribute__ ((sentinel))
• __attribute__ ((unused))