gettextize: Install also host-cpu-c-abi.m4 (needed since 2018-03-24).
* gettext-tools/m4/Makefile.am (aclocal_DATA): Install host-cpu-c-abi.m4.
* PACKAGING: Mention host-cpu-c-abi.m4.
* gettext-tools/misc/gettextize.in (m4filelist): Add host-cpu-c-abi.m4.
* gettext-tools/doc/gettext.texi (aclocal): Mention host-cpu-c-abi.m4.
