guile-gtk 1.2 update to guile 1.8

[Kevin Ryde]

I'm making some changes to guile-gtk-1.2 for guile 1.8.  The only
change actually essential is getting gtk-threads.c to use posix
threads, but I'm taking the opportunity to update to the 1.8 style
API, with compatibility macros etc for guile 1.6.

I'm feeling very inclined to drop older compatibility bits.  I'm
guessing they were for guile 1.4, but they don't really look like
they're used consistently, and configure has been demanding guile
1.5.1 or higher, so I'm guessing they haven't been run for a while.  I
think I'll end up removing bits that are no longer used.

This will be for a guile-gtk 0.5 release, so if anyone's got new stuff
they want in then this is the time to pipe up.