build: Move a .m4 file to where it belongs.
* autogen.sh: Pass '-I m4' when generating gettext-runtime/intl/configure.
* gettext-runtime/intl/m4/intdiv0.m4: Moved here from
gettext-runtime/m4/intdiv0.m4.
* gettext-runtime/intl/Makefile.am (ACLOCAL_AMFLAGS): Add '-I m4'.
(EXTRA_DIST): Add m4/intdiv0.m4.
* gettext-runtime/m4/Makefile.am (EXTRA_DIST): Remove intdiv0.m4.
