[gtksourceview/wip/modern-autotools] build: use AX_IS_RELEASE

[Sébastien Wilmet <swilmet src gnome org>]

commit 655bac400e0e8129f2cb4b0d1191285ae35220ed
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun Aug 9 13:20:33 2015 +0200

    build: use AX_IS_RELEASE
    
    https://wiki.gnome.org/Projects/GnomeCommon/Migration

 configure.ac |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 56f7596..4d3eb20 100644
--- a/configure.ac
+++ b/configure.ac
@@ -13,6 +13,8 @@ AC_INIT([gtksourceview],
        [gtksourceview],
        [https://wiki.gnome.org/Projects/GtkSourceView])
 
+AX_IS_RELEASE([git-directory])
+
 # Libtool versioning
 #
 # For development releases, keep the same numbers.