YQ2_* options

YQ2_SYSTEMWIDE and YQ2_SYSTEMDIR working well
libvorbis and libjpeg added to dependencies
Conditional compilation and definitions
Separate shared common source from the rest

Game DLL only uses shared sources for itself, so it necessary to
separate those source files from the rest.
Target names different from executable names

Targets will be 'client', 'server' and 'game', their respective
executable names will vary, but it won't affect target names. We can
'make client', 'make server', 'make game' safely ...
FindVORBIS.cmake

This cmake module dones't seem to be on standard cmake installations, so
it is necessary to add it manually to the local module location. This
module was taken from another project I found.