gmacros: Fix G_GNUC_* attributes on clang-cl
Currently when GLib is being compiled with clang-cl, clang-cl emits a lot of warnings about unused functions and variables: [583/750] Compiling C object gio/tests/modules/testmoduleb.dll.p/test-module-b.c.obj ../gio/tests/modules/test-module-b.c(39,1): warning: unused function 'test_b_get_instance_private' [-Wunused-function] G_DEFINE_TYPE (TestB, test_b, G_TYPE_OBJECT) ^ ..\gobject/gtype.h(1603,43): note: expanded from macro 'G_DEFINE_TYPE' #define G_DEFINE_TYPE(TN, t_n, T_P) G_DEFINE_TYPE_EXTENDED (TN, t_n, T_P, 0, {}) ^ ..\gobject/gtype.h(1762,60): note: expanded from macro 'G_DEFINE_TYPE_EXTENDED' #define G_DEFINE_TYPE_EXTENDED(TN, t_n, T_P, _f_, _C_) _G_DEFINE_TYPE_EXTENDED_BEGIN (TN, t_n, T_P, _f_) {_C_;} _G_DEFINE_TYPE_EXTENDED_END() ^ ..\gobject/gtype.h(2034,3): note: expanded from macro '_G_DEFINE_TYPE_EXTENDED_BEGIN' _G_DEFINE_TYPE_EXTENDED_BEGIN_PRE(TypeName, type_name, TYPE_PARENT) \ ^ ..\gobject/gtype.h(1989,24): note: expanded from macro '_G_DEFINE_TYPE_EXTENDED_BEGIN_PRE' static inline gpointer \ ^ <scratch space>(104,1): note: expanded from here test_b_get_instance_private ^ 1 warning generated. This change removes all "unused" warnings, and GLib still compiles in both `c_std=gnu99` (with `G_STATIC_ASSERT` fix) and `c_std=gnu11` mode.