[evolution] Clarify that libcanberra-gtk3 is optional.

[Matthew Barnes <mbarnes src gnome org>]

commit 09fb1eb5151f551210ef4848463dc4caabf11911
Author: Matthew Barnes <mbarnes redhat com>
Date:   Mon Jun 18 10:42:10 2012 -0400

    Clarify that libcanberra-gtk3 is optional.

 configure.ac |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 5f52ae0..87ac027 100644
--- a/configure.ac
+++ b/configure.ac
@@ -279,7 +279,10 @@ AC_ARG_ENABLE([canberra],
 	[enable_canberra="$enableval"], [enable_canberra=yes])
 
 if test $enable_canberra = yes; then
-	PKG_CHECK_MODULES([CANBERRA],[libcanberra-gtk3 >= libcanberra_gtk_minimum_version])
+	PKG_CHECK_MODULES([CANBERRA],[libcanberra-gtk3 >= libcanberra_gtk_minimum_version],,
+	[AC_MSG_ERROR([libcanberra-gtk3 not found (or version < libcanberra_gtk_minimum_version),
+	If you want to disable support for event sounds,
+	please append --disable-canberra to configure.])])
 	AC_DEFINE(HAVE_CANBERRA, 1, [Define if using Canberra and Canberra-GTK for sound])
 fi
 AC_SUBST(CANBERRA_CFLAGS)