Check if config.h passed correctly via gcc -include

configure.in

@@ -20,6 +20,7 @@ AC_INIT(strongSwan,5.0.0)
 AM_INIT_AUTOMAKE(tar-ustar)
 AC_CONFIG_MACRO_DIR([m4/config])
 AC_CONFIG_HEADERS([config.h])
+AC_DEFINE([CONFIG_H_INCLUDED], [], [defined if config.h included])
 PKG_PROG_PKG_CONFIG
 
 dnl =================================

src/libstrongswan/library.h

@@ -67,6 +67,10 @@
 #ifndef LIBRARY_H_
 #define LIBRARY_H_
 
+#ifndef CONFIG_H_INCLUDED
+# error config.h not included, pass "-include [...]/config.h" to gcc
+#endif
+
 #include "printf_hook.h"
 #include "utils.h"
 #include "chunk.h"