unistd.h is not needed
Move ASS_Shaper declaration to ass_shaper.h
Explicitly include ass_utils.h everywhere (and not config.h)
Fully fix compilation with MSVC/ICL: M_PI, strcasecmp, strings.h

As before, this does not add any build system support:
a config.h file and a project must still be manually created
(or the compiler can be run manually instead of using a project).