build: config.in.h.

Historically we used config.hin (where everybody else used
config.h.in) to please DOS.  Now that we use gnulib, there are already
tons of files with several dots, especially *.in.h.

* configure.ac: Rename config.hin as config.in.h.

lib/.gitignore

@@ -37,7 +37,7 @@
 /closeout.h
 /config.charset
 /config.h
-/config.hin
+/config.in.h
 /configmake.h
 /dirname-lgpl.c
 /dirname.c