[polari] build: Explicitly require APPSTREAM_XML

[Florian Müllner <fmuellner src gnome org>]

commit a7da80a31a997c1548ca8a626902672a078fa38c
Author: Florian Müllner <fmuellner gnome org>
Date:   Fri Sep 25 14:28:34 2015 +0200

    build: Explicitly require APPSTREAM_XML
    
    If the macro is not available, we currently end up with invalid
    Makefiles and fail with a cryptic error during the build. It is
    better to fail upfront with a clear error message, so do that
    instead.

 configure.ac |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index fb30793..ddde2ed 100644
--- a/configure.ac
+++ b/configure.ac
@@ -48,6 +48,7 @@ AC_SUBST(GLIB_COMPILE_RESOURCES)
 
 AC_PATH_PROG(DESKTOP_FILE_VALIDATE, desktop-file-validate)
 
+AX_REQUIRE_DEFINED([APPSTREAM_XML])
 APPSTREAM_XML
 
 AC_CONFIG_FILES([