C-plugin catchup

As-is, this pull request is incomplete, with the additional necessary fixes (for GNUmakefile, porting changes from the standard AviSynth plugin to the C plugin) are contained in the travisfix branch.

Of which, should the C plugin branch even its own Travis configuration?  Generally, if the master branch works, the C plugin branch will as well (but I did go ahead and modify the .travis.yml file that exists on the C plugin branch so it *should* work...not that I've actually tested it, since it seems to require the parent repo to have it in the branch first for pulls to register against it).

I can just add those commits to this pull request, but I kept them separate because it was easier to keep the cplugin_master migration branch in sync with upstream if I did.