build: add '--enable-libmpv-static' option

Signed-off-by: wm4 <wm4@nowhere>
gl_lcms: use thread-safe lcms API, require lcms2 2.6

The error log callback was not thread-safe and not library-safe. And
apparently there were some other details that made it not library-safe,
such as a global lcms plugin registry.

Switch the the thread-safe API provided by lcms2 starting with 2.6.
Remove our approximate thread-safety hacks.

Note that lcms basically provides 2 APIs now, the old functions, and
the thread-safe alternatives whose names end with THR. Some functions
don't change, because they already have a context of some sort. Care
must be taken not to accidentally use old APIs.