[fluxus] scons fixes

[Dave Griffiths]

I've fixed the guile 1.8 configure check in the SConstruct, and added some
options to the scons build:

X11=1          : Build with the X11 libraries - rather than Xorg which is
                 default.

MULTITEXTURE=0 : Multitexturing is checked at configure time, but it can
cause
                 crashes on some drivers - set this to disable the
                 multitexturing code manually.

Hope I haven't broken the Mac build, could somebody check?

cheers,

dave