Fix shared builds with MinGW

Otherwise, only the VapourSynth symbols get exported.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Merge pull request #119 from dwbuiten/exports

Fix shared builds with MinGW
Bump shared library version

Should have been done for 2.18 since it dropped the postproc-related
functions, but oh well.
Replace INCLUDES with AM_CPPFLAGS to please automake
Remove FFMS_SetOutputFormatV

It's been deprecated for two years and the soname is changing anyway so
it's a good time to finally remove it.