[quadrapassel] Check for *-validate before using it

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 616ac28757f9cc0dd806687922bd8675357acaec
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Wed Nov 6 18:37:25 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 f312620..cbbbb7c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -27,8 +27,8 @@ PKG_CHECK_MODULES(QUADRAPASSEL, [
   clutter-gtk-1.0 >= $CLUTTER_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])
 
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 dnl Internationalization