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/autoload.cpp

@@ -1,4 +1,6 @@
 // The classes responsible for autoloading functions and completions.
+#include "config.h"  // IWYU pragma: keep
+
 #include <assert.h>
 #include <errno.h>
 #include <pthread.h>