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'
    ..\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
    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.
10 jobs for clang-cl-attributes in 26 minutes and 34 seconds (queued for 3 seconds)