enable zenity support by default if it is in the path

configure.in

@@ -458,9 +458,9 @@ AC_ARG_ENABLE(icecream,
 ,)
 
 AC_ARG_ENABLE(zenity,
-    AS_HELP_STRING([--enable-zenity],
-        [Display a icon in the notification area during build.]),
-,)
+    AS_HELP_STRING([--disable-zenity],
+        [Do not display a build icon in the notification area (on unix) during build.]),
+,enable_zenity=yes)
 
 AC_ARG_ENABLE(cl-x64,
     AS_HELP_STRING([--enable-cl-x64],
@@ -8006,8 +8006,14 @@ dnl Use zenity during build
 dnl ===================================================================
 AC_MSG_CHECKING([whether to use zenity during build])
 if test "$enable_zenity" = "yes"; then
-    ENABLE_ZENITY=TRUE
-    AC_MSG_RESULT([yes])
+   AC_PATH_PROGS( ZNTY, zenity )
+   if test -z "ZNTY"; then
+      ENABLE_ZENITY=FALSE
+      AC_MSG_RESULT([no])
+   else
+      ENABLE_ZENITY=TRUE
+      AC_MSG_RESULT([yes])
+   fi
 else
     ENABLE_ZENITY=FALSE
     AC_MSG_RESULT([no])