[easytag] Warn if APPDATA_XML macro is not available

[David King <davidk src gnome org>]

commit de1f4d9e5201d3e5511c35b7d71cba9709aa88a5
Author: David King <amigadave amigadave com>
Date:   Fri Oct 31 17:03:21 2014 +0000

    Warn if APPDATA_XML macro is not available
    
    Rather than getting an error at make time, output an error message
    during configure if the macro is not found.

 configure.ac |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 0c269f1..8b0d441 100644
--- a/configure.ac
+++ b/configure.ac
@@ -34,7 +34,8 @@ AS_IF([test "x$USE_NLS" != "xno"],
       [AC_DEFINE([ENABLE_NLS], [1], [Native Language support is enabled])])
 
 dnl AppData
-APPDATA_XML
+m4_ifdef([APPDATA_XML], [APPDATA_XML],
+         [AC_MSG_ERROR([appdata-tools is required for appdata-xml.m4])])
 
 dnl -------------------------------
 dnl Checks for programs.