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/path.cpp

@@ -1,6 +1,8 @@
 // Directory utilities. This library contains functions for locating configuration directories, for
 // testing if a command with a given name can be found in the PATH, and various other path-related
 // issues.
+#include "config.h"  // IWYU pragma: keep
+
 #include <assert.h>
 #include <errno.h>
 #include <sys/stat.h>