[gnome-tetravex] Check for *-validate before using it

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 6e7c1efdd87d4e8dcfbce9d3dd2ffe4bc174164e
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Wed Nov 6 18:42:42 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 ac4d843..0772c8f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -22,8 +22,8 @@ PKG_CHECK_MODULES(GNOME_TETRAVEX, [
   gtk+-3.0 >= $GTK_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 GResources