build: set tool paths from config arguments

This will make it easier to e.g. integrate with Git on Windows, since
you can specify the path to Git rather than messing with the environment
variables.
windows: fix floating point classifiers
windows: update DirectSound audio code
windows: update Windows front end
windows: remove superfluous glClear
windows: cap frame rate the easy way
build: add LibCURL module
sdl: call sg_game_destroy()
core: expand interface to version information
sdl: close window immediately on shutdown
windows: fix missing header
sdl: cap frame rate at 200 fps