Simplify PO build rules for our NSIS-only useto support VPATH.
* configure.ac (AC_CONFIG_FILES): Remove build-cc. * po/Makefile.in.in (BUILD_CC): New. (.po.mo): Make rule a dummy. (.po.gmo): Ditto. * po/Rules-nsis (DISTFILES.extra1): Remove build-cc.in. (nsis-msgfmt): USe BUILD_CC instead of the reading build-cc. (%.nsi:%.po): Fix for VPATH build. * po/build-cc.in: Remove.