intl: turn LIBINTL into -L / -l form

This variable currently refers directly, not to a .la file, but to an .a
file.  This produces wrong results when building into a library on some
platforms: so convert it to the general form "-L${top_builddir}../intl
-lintl ..." ... so that both libtool and non-libtool builds will always
do the right thing for both static and shared links.

intl/ChangeLog
2021-02-04  Nick Alcock  <nick.alcock@oracle.com>

	* configure.ac (LIBINTL): Transform into -L/-lintl form.
	* configure: Regenerate.