fail if avahi is not found

If I configure with --enable-avahi, presumably I want the code to be
built with avahi support...

configure.ac

@@ -11700,11 +11700,9 @@ dnl Test whether to use avahi
 dnl ===================================================================
 if test "$_os" != "WINNT" -a "$_os" != "Darwin" -a "$enable_avahi" = "yes"; then
     PKG_CHECK_MODULES([AVAHI], [avahi-client >= 0.6.10],
-                      [ENABLE_AVAHI="TRUE"], [ENABLE_AVAHI=""])
-    if test "$ENABLE_AVAHI" = "TRUE"; then
+                      [ENABLE_AVAHI="TRUE"])
     AC_DEFINE(HAVE_FEATURE_AVAHI)
     AVAHI_CFLAGS=$(printf '%s' "$AVAHI_CFLAGS" | sed -e "s/-I/${ISYSTEM?}/g")
-    fi
 fi
 
 AC_SUBST(ENABLE_AVAHI)