Fixed out-of-source-tree builds for 'po'

po/Makefile.in.in

@@ -496,7 +496,7 @@ update-gmo: Makefile $(GMOFILES)
 	@:
 
 #
-# The next two targets are customized for MHD
+# The next three targets are customized for MHD
 #
 
 # Recreate Makefile by invoking config.status. Explicitly invoke the shell,
@@ -510,6 +510,10 @@ Makefile: Makefile.in.in Makevars $(top_builddir)/po-config.status @POMAKEFILEDE
 $(top_builddir)/po-config.status: $(srcdir)/po-configure.ac.in $(top_srcdir)/configure.ac
 	cd $(top_builddir) && $(MAKE) $(AM_MAKEFLAGS) po-config.status
 
+remove-potcdate.sed: $(srcdir)/remove-potcdate.sin
+	$(SED) -e '/^#/d' '$(srcdir)/remove-potcdate.sin' > t-$@
+	mv t-$@ $@
+
 force:
 
 # Tell versions [3.59,3.63) of GNU make not to export all variables.