libwebsockets

windows: pthreads

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.