Add support for external pthreads lib on windows and some docs about how to do.
It can build with LWS_WITH_THREADPOOL and LWS_WITH_MINIMAL_EXAMPLES including the
pthreads-dependent ones without warnings or errors on windows platform as well with this.
pthreads_t can be anything, including a struct - not a pointer-to-a-struct
but the struct itself. These can't be cast to a void * for printing as they can
on linux, where the base type is a pointer.
Let's fix all the usage of those to determine their own thread index in terms
of the meaning to the program rather than as a tid.
https://libwebsockets.org/pipermail/libwebsockets/2019-April/007937.html
thanks to Bruce Perens for noting it.
This doesn't change the intention or status of the CC0 files, they were
pure CC0 before (ie, public domain) and they are pure CC0 now. It just
gets rid of the (C) part at the top of the dedication which may be read
to be a bit contradictory since the purpose is to make it public domain.
