Really minimal modification for Linux integration

There was an special intention on making this as less intrusive as possible.

There was no unix_compat.h, so i based it on macos_compat.h, some function definition/declaration missmatches so in really was buggy (forced to fix), mkdir() had 2 arguments but a define with only 1 was in place (hence forced to fix), filelength was not needed only on macos (hence fix), some flags, some libs, really super simple minimal Makefiles (with clean) and works.

Please, even if you are long time retired on this project, i hope you got at least 30 min to check and accept the request. :)

Thank you.

juanmabc "Juan Manuel Borges Caño <juanmabcmail@gmail.com>"