fatal error: 'config.h' file not found

Version 4.2.3 installs xc.h referencing config.h which isn't installed.

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

config.h can be a commonly available header, and HAVE_CONFIG_H can easily be defined by the project itself, unrelated to libxc.

Particularly, bagel-1.1.2.21 fails which is a regression with libxc-4.2.3:

In file included from ../../src/grad/gradeval.h:30:
In file included from ../../src/scf/ks/ks.h:30:
In file included from ../../src/scf/ks/xcfunc.h:35:
/usr/local/include/xc.h:17:10: fatal error: 'config.h' file not found
#include "config.h"
         ^~~~~~~~~~
In file included from get_grad.cc:32:
In file included from ../../src/opt/opt.h:34:
In file included from ../../src/grad/gradeval.h:30:
In file included from ../../src/scf/ks/ks.h:30:
In file included from ../../src/scf/ks/xcfunc.h:35:
/usr/local/include/xc.h:17:10: fatal error: 'config.h' file not found
#include "config.h"
         ^~~~~~~~~~