Fix gtk+ and jack automagic detection.

https://github.com/calf-studio-gear/calf/issues/166

--- a/configure.ac
+++ b/configure.ac
@@ -150,13 +150,35 @@
   [set_enable_sse="no"])
 AC_MSG_RESULT($set_enable_sse)
 
+AC_MSG_CHECKING([whether to enable GUI])
+AC_ARG_ENABLE(gui,
+  AS_HELP_STRING([--enable-gui],[enable graphical user interface]),
+  [set_enable_gui=$enableval],
+  [set_enable_gui=yes])
+AC_MSG_RESULT($set_enable_gui)
+
+AC_MSG_CHECKING([whether to enable JACK])
+AC_ARG_ENABLE(jack,
+  AS_HELP_STRING([--enable-jack],[enable support for JACK]),
+  [set_enable_jack=$enableval],
+  [set_enable_jack=yes])
+AC_MSG_RESULT($set_enable_jack)
+
 ############################################################################################
 # Compute status shell variables
 
-if test "$GUI_ENABLED" = "yes" -a "$JACK_FOUND" = "yes"; then
+if test "x$set_enable_gui" = "xno"; then
+  GUI_ENABLED="no (disabled by user choice)"
+fi
+
+if test "$JACK_FOUND" = "yes"; then
   JACK_ENABLED="yes"
 fi
 
+if test "x$set_enable_jack" = "xno"; then
+  JACK_ENABLED="no (disabled by user choice)"
+fi
+
 if test "$GUI_ENABLED" = "yes" -a "$LV2_ENABLED" = "yes"; then
   LV2_GUI_ENABLED="yes"
 fi