[tali] Check for *-validate before using it

[Michael Catanzaro <mcatanzaro src gnome org>]

commit c9000a5d9d3faeb3063dbe9d42b229ba762fe691
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Wed Nov 6 18:42:24 2013 -0600

    Check for *-validate before using it

 configure.ac |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 300fef0..4a2f4a6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -66,8 +66,8 @@ PKG_CHECK_MODULES(TALI, [
   librsvg-2.0 >= $RSVG_REQUIRED
 ])
 
-AC_PATH_PROG(APPDATA_VALIDATE, appdata-validate)
-AC_PATH_PROG(DESKTOP_FILE_VALIDATE, desktop-file-validate)
+AC_PATH_PROG([APPDATA_VALIDATE], [appdata-validate], [/bin/true])
+AC_PATH_PROG([DESKTOP_FILE_VALIDATE], [desktop-file-validate], [/bin/true])
 
 dnl ###########################################################################
 dnl Internationalization