tests: add PTHREAD_CFLAGS for make check to pass on OS X
tests: add PTHREAD_CFLAGS for make check to pass on OS X

Currently "make check" fails with:
"error: argument unused during compilation: '-pthread'"

PTHREAD_CFLAGS now contains -Qunused-arguments to fix that.
Explanation here: http://savannah.gnu.org/patch/?8186#comment21

https://bugzilla.gnome.org/show_bug.cgi?id=747954

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer