1. Updated to the most recent copy of ax_pthread.m4.
2. Removed the darwin-clang check m4.
3. Added a check to see if AX_PTHREAD added the flag `-Qunused-arguments` for clang and if so prepend `-Xcompiler` so libtool will use it. Otherwise when building on Sierra's clang you get "soft" warnings on the build of the dylib.