Fixes to make it build on COSI's Ubuntu machines.

The version of GCC they were running didn't like me directly
assigning to members of anonymous unions in struct initializers
(see `grafix/entity.c') and a weird problem w/ <setjmp.h> and
<png.h> was breaking the build. Also, removed silly dependency on
GLEW and hacked together a Linux-friendly fix for the problem where
`pkg-config --cflags gl' doesn't actually give a path to <gl.h>.
Tidied up `main.c' a bit.

It's still pretty ugly, but the render is made a bit smoother by
changing the frame interval to 50ms rather than 10ms. Also, some of
the stuff before the main loop is just a touch better looking.