Add config.rpath for AM_GNU_GETTEXT

Commit e8864b1b28c2 ("config: libintl/libiconv for gettext() detection")
added an empty config.rpath with a comment that the real one doesn't
work with libtool.

However, an empty config.rpath doesn't really work: eg. on FreeBSD,
where libintl is in /usr/local/lib, configure thinks that gettext
doesn't exist and NLS should be disabled, which currently isn't
supported in the source, and hence requires manual workaround to
directly link -lintl without relying on configure. config.rpath is
essential to let it be detected either in --prefix or using
--with-libintl-prefix.

I also don't see the mentioned issue with libtool flags applied to
compilation, it seems to work fine to pass LTLIBINTL to libtool. It's
unnecessary to include LTLIBICONV as the configure test will
automatically append that to LTLIBINTL if it is necessary to link with
libiconv.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10538