build: Remove redundant .m4 file.
* autogen.sh (GNULIB_MODULES_LIBINTL): Add iconv.
* gettext-runtime/m4/iconv.m4: Remove file.
* gettext-runtime/m4/Makefile.am (EXTRA_DIST): Remove it.
* Makefile.am (distcheck-hook): Don't check it.
* gettext-tools/m4/Makefile.am (aclocal_DATA): Take it from the
gettext-runtime/gnulib-m4/ directory.
