patch 8.1.1149: building desktop files fails with older msgfmt

Problem:    Building desktop files fails with older msgfmt.
Solution:   Add autoconf check.  Avoid always building the desktop files.
diff --git a/src/config.mk.in b/src/config.mk.in
index b5d1ebd..e20a10f 100644
--- a/src/config.mk.in
+++ b/src/config.mk.in
@@ -169,6 +169,7 @@
 MAKEMO		= @MAKEMO@
 
 MSGFMT		= @MSGFMT@
+MSGFMT_DESKTOP	= @MSGFMT_DESKTOP@
 
 # Make sure that "make first" will run "make all" once configure has done its
 # work.  This is needed when using the Makefile in the top directory.