[easytag/wip/yelp-m4] Error out from configure if yelp.m4 is not found

[David King <davidk src gnome org>]

commit 48b46925b297efc2b0b9f3d2e884d07ad7096d91
Author: David King <amigadave amigadave com>
Date:   Sun Nov 3 10:36:43 2013 +0000

    Error out from configure if yelp.m4 is not found

 configure.ac |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index b18e4fd..5a2248a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -78,7 +78,8 @@ AS_IF([test "x$have_desktop_file_validate" = "xyes"],
 AM_CONDITIONAL([EASYTAG_ENABLE_TESTS], [test "x$testing_utilities" = "xyes"])
 
 dnl Help.
-YELP_HELP_INIT
+m4_ifdef([YELP_HELP_INIT], [YELP_HELP_INIT],
+         [AC_MSG_ERROR([yelp-tools is required to build EasyTAG help])])
 
 dnl -------------------------------
 dnl Handle cross-compiling with mingw32.