Partial support for building the VapourSynth plugin for Linux (maybe OSX, but that's completely untested here):
Fix permissions on configure
Linux compilation support
It just took some adjusting of configure and GNUmakefile; the shared object settings were derived from what the FFMS2 C-plugin does when compiling. 'Partial' because the install rules are still missing, but it does generate a valid .so that Linux-native VapourSynth can use. Windows use needs to be tested to make sure nothing broke, even though compilation still succeeds.
...and I just now realized I hadn't shifted the LDFLAGS section's if block from 'MINGW' to 'WIN32'.