Work around OS X GTK treating Option as an AltGr key.

If I'm using Option as the Meta key, I want to suppress OS X GTK's
default behaviour of treating it as an AltGr-oid which changes the
keyval and Unicode translation of alphabetic keys. So on OS X I enable
a somewhat bodgy workaround which retranslates from the hardware
keycode as if the Option modifier had not been active at the time, and
use that as the character to prefix Esc to.

This is a bit nasty because I have to hardwire group = 0 in the call
to gdk_keymap_translate_keyboard_state(), whereas in principle what I
wanted was group = (whatever would have resulted from everything else
in the key event other than MOD1). However, in practice, they seem to
be the same, so this will do for the moment.

unix/unix.h

@@ -25,6 +25,8 @@
 #define NO_PTY_PRE_INIT /* OS X gets very huffy if we try to set[ug]id */
 #define SET_NONBLOCK_VIA_OPENPT /* work around missing fcntl functionality */
 #define OSX_META_KEY_CONFIG /* two possible Meta keys to choose from */
+/* this potential one of the Meta keys needs manual handling */
+#define META_MANUAL_MASK (GDK_MOD1_MASK)
 #endif
 
 struct Filename {