build: Fix "make distcheck" after the line-breaking algorithm changed.
* gettext-runtime/po/Makefile.in.in (.po.gmo, $(POFILES)): In the gettext-tools
package, when not cross-compiling, use the locally built msg* programs.
* gettext-tools/configure.ac: When not cross-compiling, set MSGMERGE and
MSGMERGE_FOR_MSGFMT_OPTION to point to locally built msgmerge program.
