[gnome-klotski] Check for *-validate before using them

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 920600d129f7f0d0538efa957350a63ab08c55ad
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Wed Nov 6 18:30:54 2013 -0600

    Check for *-validate before using them

 configure.ac |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 9f69193..19c7af3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -21,8 +21,8 @@ PKG_CHECK_MODULES(GNOME_KLOTSKI, [
   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