meson: Document why -Wimplicit-fallthrough is not used with clang

This changes the macro to ensure proper fallthrough attributes are
used with clang in case this option is added in the future.

meson.build

@@ -351,6 +351,9 @@ possible_common_cc_flags = [
         '-Werror=shift-overflow=2',
         '-Werror=undef',
         '-Wfloat-equal',
+        # gperf prevents us from enabling this because it does not emit fallthrough
+        # attribute with clang.
+        #'-Wimplicit-fallthrough',
         '-Wimplicit-fallthrough=5',
         '-Winit-self',
         '-Wlogical-op',

src/fundamental/macro-fundamental.h

@@ -40,7 +40,7 @@
 #  define _alloc_(...) __attribute__((__alloc_size__(__VA_ARGS__)))
 #endif
 
-#if __GNUC__ >= 7
+#if __GNUC__ >= 7 || __clang__
 #  define _fallthrough_ __attribute__((__fallthrough__))
 #else
 #  define _fallthrough_