add (or restore) config.h to all files

The autoconf-generated config.h contains a number of directives which
may alter the behaviour of system headers on certain platforms. Always
include it in every C++ file as the first include.

Closes #2993.

src/parse_productions.cpp

@@ -1,3 +1,5 @@
+#include "config.h"  // IWYU pragma: keep
+
 #include <stdio.h>
 
 #include "common.h"