Fixed .la dependency tracking for -lpthread

Fixed .la dependency tracking for -lpthread

With the recent change, adding THREAD_LIBRARY_LIBS to handle the optional
-lpthread flag, it changed the success argument to AC_CHECK_LIB([pthread])
which changed its default behaviour.

AC_CHECK_LIB's default success behaviour is to prepend the flag to LIBS,
and define HAVE_LIBPTHREAD.

Without -lpthread in LIBS, libtool's automatic handling of library
dependencies (via .la files) stopped working, and became much more irregular.

Since nearly all of Barry relies on libusb, which also relies on libpthread
on Linux, all of Barry also relies on libpthread.  Therefore it makes
sense to retain the default LIBS setting, and get rid of all the hardcoded
-lpthread and the THREAD_LIBRARY_LIBS flags as well.