lib/getopt*.c: Include <config.h> only HAVE_CONFIG_H is defined.

The CMake-based build doesn't use config.h.

Up-to-date getopt_long in Gnulib is LGPLv2 so at some
point it could be included in XZ Utils too but for now
this commit is enough to make CMake-based build possible.

lib/getopt.c

@@ -21,7 +21,9 @@
    Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
 
 #ifndef _LIBC
+# ifdef HAVE_CONFIG_H
 #  include <config.h>
+# endif
 #endif
 
 #include "getopt.h"

lib/getopt1.c

@@ -20,7 +20,9 @@
 #ifdef _LIBC
 # include <getopt.h>
 #else
+# ifdef HAVE_CONFIG_H
 #  include <config.h>
+# endif
 # include "getopt.h"
 #endif
 #include "getopt_int.h"