build: replace intltool with gettext
Gettext, since version 0.18, provides infrastructure equivalent to intltool (generating po/Makefile and translating .desktop files or .xml).
Also, it seems to work better in that it actually honors MSGFMT_OPTS that are determined by the configure script. That one typically ends up set to "-c", meaning "check". Without it the bugs in the .po files are easy to overlook.