multiple include guard for config_global.h

Strictly speaking not necessary, but it should prevent compilers
from needlessly opening the file repeatedly.

config/config_global.h.in

@@ -9,8 +9,13 @@ Any change in this header will cause a rebuild of almost everything.
 
 */
 
+#ifndef CONFIG_GLOBAL_H
+#define CONFIG_GLOBAL_H
+
 #undef HAVE_CXX11_DELETE
 #undef HAVE_GCC_BUILTIN_ATOMIC
 #undef HAVE_SFINAE_ANONYMOUS_BROKEN
 #undef HAVE_THREADSAFE_STATICS
 #undef HAVE_SYSLOG_H
+
+#endif