Make Android.mk threads sensitive.

Make the definition of CERES_NO_THREADS conditional on the presence
of the CERES_HAVE_PTHREADS define.

This allows the user to enable threading in ceres using a combination
of CERES_EXTRA_DEFINES and linker flags which they need to do in
their app.

Change-Id: I43b99fc829001166e2f6dfb0a770768e61470591