[libc] avoid clang complains on unknown attributes when parsing libc

especially useful for frama-clang, but could come handy should a user want
to use clang as C pre-processor

share/libc/features.h

@@ -34,12 +34,27 @@
 #define __POP_FC_STDLIB
 #endif
 
+#ifdef __clang__
+# define __CLANG_IGNORE_ATTRS_PUSH__ \
+_Pragma("clang diagnostic push") \
+_Pragma("clang diagnostic ignored \"-Wunknown-attributes\"")
+# define __CLANG_IGNORE_ATTRS_POP__ \
+_Pragma("clang diagnostic pop")
+#else
+# define __CLANG_IGNORE_ATTRS_PUSH__
+# define __CLANG_IGNORE_ATTRS_POP__
+#endif
+
 #ifdef	__cplusplus
-# define __BEGIN_DECLS	extern "C" {
-# define __END_DECLS	}
+# define __BEGIN_DECLS \
+extern "C" { \
+  __CLANG_IGNORE_ATTRS_PUSH__
+# define __END_DECLS \
+__CLANG_IGNORE_ATTRS_POP__ \
+}
 #else
-# define __BEGIN_DECLS
-# define __END_DECLS
+# define __BEGIN_DECLS __CLANG_IGNORE_ATTRS_PUSH__
+# define __END_DECLS __CLANG_IGNORE_ATTRS_POP__
 #endif
 
 #undef __LEAF