C: optimize dirty pixel search.

Now processes the image four bytes at a time rather than one, which
results in a fair amount of speedup.

Big thanks to @Youka for suggestions and an example implementation.
Hopefully I didn't screw it up.
C: Change checkLine signature.

Results are now passed in as a pointer and the return value is a status
code.

Also the results array now contains length+1 items. The extra index is
reports whether or not the line is invisible for its whole duration.
Makefile: attempt to tell if being run on Windows.

Stolen shamelessly from stack overflow.