Change how configuration works.

  * move win32/config0.h -> config/config_windows0.h
  * move win32/stdint.h -> msvc/stdint.h
  * move posix/defines.h -> config/config_unix.h
  * add config.h - brings in config_(windows|unix).h as required
  * add config.h to *.cpp
  * Self-contain FFmpegSource2 with required function defines wrapped with
    __UNIX__ instead of the now removed defines.h.
  * Edit aegisub_vs2008.vcproj to remove explicit inclusion of config.h, also
    change VCPreBuildEventTool to create windows_config.h from
    windows_config0.h.  Add msvc to include path.
  * Change configure to create ./acconf.h instead of posix/acconf.h

This will allow us to create a more standard and platform agnostic way of
configuration aegsisub during build time.

Originally committed to SVN as r2621.