Fix define check for GCC symbol visibility
Fix define check for GCC symbol visibility

Define the __attribute__ symbol visibility (for GCC) by checking
for GCC itself, instead of different OSes (which might also have
different compilers).

Clang should define __GNUC__ as well for GCC compatibility, so this
should cause no regressions (while fixing the symbols visibility on any
OS which uses GCC).

AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO,AcademySoftwareFoundation/OpenColorIO