Some GCC-compatibility definitions. Define clang's feature test pseudomacros

to always evaluate to 0 if we are using a compiler that doesn't implement them.
This lets us use the macros easily in standard headers (e.g. stdatomic.h, which
should be the subject of my next commit).

Approved by:	dim (mentor)

sys/sys/cdefs.h

@@ -617,4 +617,14 @@
 #endif
 #endif
 
+#ifndef	__has_feature
+#define	__has_feature(x) 0
+#endif
+#ifndef	__has_include
+#define	__has_include(x) 0
+#endif
+#ifndef	__has_builtin
+#define	__has_builtin(x) 0
+#endif
+
 #endif /* !_SYS_CDEFS_H_ */