Remove left over lock file from intltool on "make distcheck"

Debian and ArchLinux (and maybe more) patched their "intltool" package
to use a lock file. But this lock file is not removed after its use and
so "make distcheck" complains about it.

Makefile.am

@@ -35,6 +35,12 @@ DISTCLEANFILES = \
 uninstall-local:
 	rm -rf $(DESTDIR)$(pkgdatadir);
 
+# Remove po/.intltool-merge-cache.lock on "distcheck" which is left over by various incorrectly
+# patched versions of intltool, e.g. https://bugs.archlinux.org/task/67098 or
+# https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=991623
+distclean-local:
+	rm -f $(top_builddir)/po/.intltool-merge-cache.lock
+
 # manually install some files under another name
 install-data-local:
 	$(mkinstalldirs) $(DESTDIR)$(pkgdatadir)