[Gtkradiant] build system for OSX port

[Timothee Besset]

I am cross-posting this to the regular GtkRadiant list as well, since
people there have been making some effort regarding new scons build
system.

There is ongoing effort to port GtkRadiant to OSX 10.2, using the new
hardware acceleration capabilities of XDarwin. I believe the cons build
system for GtkRadiant can be extended to OSX building. This is going to
require a cleaner configuration step, detecting the OS and selecting
different base CFLAGS etc.

There has been work on a brand new scons-based build system for GtkRadiant
also (scons is a next-gen version of cons, which relies on Python instead
of Perl, and an overall cleaner design than cons is). I'm not clear what
the status on scons is, wether to try the scons build system for OSX or
not.

1 - using scons: have to cope with scons not being finalized yet for GtkR win32/Linux
2 - using cons: will have to propagate the changes to scons build system when we roll it in
I think 2 is safer, sticking to cons will help maintain focus on the OSX issues.

TTimo