Add internal abstraction for POSIX Threads

Add internal abstraction for POSIX Threads

This prepares for a Windows backend without dependency on pthreads-w32.

pthread_* is renamed to usbi_* and PTHREAD_* to USBI_*.

A usbi_mutex_static_t and usbi_mutex_static_lock() and _unlock() are
introduced for statically initialized mutexes, since they may be
implemented using other types when pthreads mutexes aren't used.

Move -pthread from libusb/Makefile.am to host-specific THREAD_CFLAGS in
configure.ac. This will enable optional use of -lpthread for cygwin.

[dsd: minor tweaks, and roll in a change based on patches from Pete
Batard to only build dpfp_threaded example when we're using pthreads]