GTK 2 doesn't _documentedly_ provide a helpful compile option to let
us check this one in advance of GTK 3, but you can fake one anyway by
compiling with -D__GDK_KEYSYMS_COMPAT_H__, so that gdkkeysyms-compat.h
will believe that it's already been included :-) We now build cleanly
under GTK 2 with that predefine.
This is less than ideal - passphrase input now happens in ISO 8859-1,
and the passphrase prompt window is neither centred nor always-on-top.
But it basically works, and restores bare-minimum GTK 1 support to the
codebase as a whole.
