This pollutes the reserved compiler namespace and breaks applications
trying to do their own feature detection. For instance, this falsely
detects that alloca is not a builtin on gcc:

    #include <glib.h>
    #if defined(__has_builtin)
    # if !__has_builtin(alloca)
    #  error "wtf glib?"
    # endif
    #else
    /* version-checking to determine alloca existence */
    #endif

Instead, define our own g_macro__has_* versions that have the
behaviour that we need.

https://bugzilla.gnome.org/show_bug.cgi?id=794635