On OS X, be able to configure either Option or Command as Meta.

Personally I like using Command as the Esc-prefixing Meta key in
terminal sessions, because it occupies the same physical keyboard
position as the Alt key that I'm used to using on non-Macs. OS X
Terminal uses Option for that purpose (freeing up Command for the
conventional Mac keyboard shortcuts, of course), so I anticipate
differences of opinion.

Hence, here's a pair of OSX-specific config options which permit a
user to set either, or neither, or both of those modifier keys to
function as the terminal Meta key.

putty.h

@@ -751,6 +751,8 @@ void cleanup_exit(int);
     X(INT, NONE, erase_to_scrollback) \
     X(INT, NONE, compose_key) \
     X(INT, NONE, ctrlaltkeys) \
+    X(INT, NONE, osx_option_meta) \
+    X(INT, NONE, osx_command_meta) \
     X(STR, NONE, wintitle) /* initial window title */ \
     /* Terminal options */ \
     X(INT, NONE, savelines) \