[empathy] Fail to build if --enable-gtk3 is passed but only gtk2 is available

[Xavier Claessens <xclaesse src gnome org>]

commit 701e8ee3edfb7a2de17f1d3b82137d0dfdb3388f
Author: Xavier Claessens <xclaesse gmail com>
Date:   Thu Jul 8 15:24:10 2010 +0200

    Fail to build if --enable-gtk3 is passed but only gtk2 is available

 configure.ac |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 0de76f4..e004410 100644
--- a/configure.ac
+++ b/configure.ac
@@ -177,6 +177,10 @@ else
   have_gtk3="no"
 fi
 
+if test "x$enable_gtk3" = "xyes" -a "x$have_gtk3" != "xyes"; then
+   AC_MSG_ERROR([Couldn't find gtk3 dependencies.])
+fi
+
 if test "x$have_gtk3" == "xyes"; then
   # GTK3 specific deps
   PKG_CHECK_MODULES(CANBERRA, libcanberra-gtk3 >= $LIBCANBERRA_GTK_REQUIRED)