HEAD catchup + better install/pkgconfig parity with master

Considering the recent VS header licensing stuff, I figured this needed updating too.

It also allows c_plugin to match master in its library naming and pkg-config stuff.  A side effect of this is that the c_plugin branch can be used to provide x264's FFMS support when using static builds of libffms2 under MinGW, since this is broken in master (regression in https://github.com/FFMS/ffms2/commit/7e11704ab32daffcd10c8e7f276d9e1ec147027e - it also broke the C-plugin despite being a shared library, which is why I'd already reverted that in c_plugin).