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_execution.cpp

@@ -6,6 +6,8 @@
 //
 // Non-fatal errors are printed as soon as they are encountered; otherwise you would have to wait
 // for the execution to finish to see them.
+#include "config.h"  // IWYU pragma: keep
+
 #include <assert.h>
 #include <errno.h>
 #include <stdarg.h>