[libc][cdefs] add a few additional features in cdefs

This seems to only be needed for older gccs (found with 7.5.0).

lib/libc/include/sys/cdefs.h

@@ -56,6 +56,25 @@
 #define __BEGIN_DECLS __BEGIN_CDECLS
 #define __END_DECLS __END_CDECLS
 
+/*
+ * Testing against Clang-specific extensions.
+ */
+#ifndef __has_attribute
+#define __has_attribute(x)  0
+#endif
+#ifndef __has_extension
+#define __has_extension     __has_feature
+#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
+
 /*-
  * Deal with _ANSI_SOURCE:
  * If it is defined, and no other compilation environment is explicitly