[AD] Build process progression

[Jon Rafkind <workmin@xxxxxxxxxx>]

This is in response to Evert asking me to make a list of things Ive done 
with the build process thus far. Sorry for not sending this sooner. It 
got.. lost.. in the wake of my busy week.

There is a base SConscript in 4.3/ which figures out what platform its 
on and calls a platform-specific scons file in 4.3/scons to configure 
platform specific things. The files in scons/ can be thought of as 
classes that inherit from each other. linux.scons inherits from 
unix.scons which inherits from all.scons. all.scons is the base 'class' 
that all other scons files should inherit from. Currently the following 
platform specific build files exist:

all.scons
bsd.scons
linux.scons
unix.scons

There is also a checks.py file which contains all the `configure'-like 
checks Allegro needs.

In the works:
I have an OS X build file and a mingw build file but since 4.3 doesnt 
build on either of them yet I cant test them very well.

And finally, I just checked in a first draft scons build file for the 
demo game, skater. It works well on unix but hasnt been tested on other 
platforms.