gcc-plugins: Handle unusual header environment

- Fix redefined macro error under a Darwin build host
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 Comment: Kees Cook <kees@outflux.net>
 
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Merge tag 'gcc-plugins-v5.2-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull gcc-plugins fix from Kees Cook:
 "Handle unusual header environment, fixing a redefined macro error
  under a Darwin build host"

* tag 'gcc-plugins-v5.2-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  gcc-plugins: Fix build failures under Darwin host

scripts/gcc-plugins/gcc-common.h

@@ -150,8 +150,12 @@ void print_gimple_expr(FILE *, gimple, int, int);
 void dump_gimple_stmt(pretty_printer *, gimple, int, int);
 #endif
 
+#ifndef __unused
 #define __unused __attribute__((__unused__))
+#endif
+#ifndef __visible
 #define __visible __attribute__((visibility("default")))
+#endif
 
 #define DECL_NAME_POINTER(node) IDENTIFIER_POINTER(DECL_NAME(node))
 #define DECL_NAME_LENGTH(node) IDENTIFIER_LENGTH(DECL_NAME(node))