There are plenty of macros that only check for GNU GCC and don't bother checking support from other popular compilers. For example, in libguile/__scm.h, the macros SCM_NORETURN, SCM_UNUSED, SCM_EXPECT, and many more, all only check for GCC support. Most (if not all) of these language extensions are supported by Clang, and usually using the same exact syntax!