build: Switch from intltool to gettext

See https://gitlab.xfce.org/xfce/xfce4-dev-tools/-/issues/41.

Makefile.am

@@ -8,23 +8,17 @@ SUBDIRS =								\
 desktopdir = $(datadir)/applications
 desktop_in_files = xfce4-taskmanager.desktop.in
 desktop_DATA = $(desktop_in_files:.desktop.in=.desktop)
-@INTLTOOL_DESKTOP_RULE@
+
+xfce4-taskmanager.desktop: xfce4-taskmanager.desktop.in
+	$(AM_V_GEN)$(MSGFMT) --desktop --template $< -d $(top_srcdir)/po -o $@
 
 ChangeLog: Makefile
 	ChangeLog-git.sh > $(srcdir)/ChangeLog || touch $(srcdir)/ChangeLog
 
 EXTRA_DIST =								\
-	README.md								\
-	intltool-extract.in						\
-	intltool-merge.in						\
-	intltool-update.in						\
 	$(desktop_in_files)
 
 DISTCLEANFILES =							\
-	intltool-extract						\
-	intltool-merge							\
-	intltool-update							\
-	po/.intltool-merge-cache.lock					\
 	$(desktop_DATA)
 
 # vi:set ts=8 sw=8 noet ai nocindent syntax=automake: