initial dos (djgpp) port

This is an initial quick port to dos (djgpp) platform. Only the wav writer is available yet, a real audio output driver may or may not be added later.

First commit:
- use findfirst() instead of stat() for dos in file_io.
- always use O_BINARY in file open flags, which removes a few ifdefs
  from the body of the code. define it as 0 if not already defined.
- make the lock functions nop for dos builds.
- combine some _WIN32 conditionals with __DJGPP__ where appropriate.
  complete some other missing pieces for djgpp.

Second commit:
- add djgpp-specific makefile and config.

Tested to work so far, but obviously #24 affects it very much noticeably.