gmacros.h: Complete the use of GLIB_UNAVAILABLE_MACRO

Fixes #2376 (closed).

This MR

• adds GLIB_AVAILABLE_MACRO_IN_* where needed in gmacros.h, or a comment when such use is not possible;
• fixes GLIB_UNAVAILABLE_MACRO and GLIB_DEPRECATED_MACRO_FOR so that these macros print their arguments correctly.