[Gtkradiant] HL support, some notes

[Timothee Besset]

A few notes about HL support as in the current CVS trunk. Until I have
finished some new stuff in synapse (namely XML configuration), some things
are hardcoded into the binaries. On Linux you have to build with ./cons or
./cons -- release, on win32 you have to edit include/hltoggle.h (default
on win32 is to build for HL, just edit the .h).

For now if you want to run HL, you will have to get the preliminary media
zip from zerowing: http://zerowing.idsoftware.com/stuff/HL-media-0329.zip

TTimo