More configure.ac cleanup
Switch to using ACX_PTHREAD rather than our own autoconf code.
Update the acx_pthread.m4 file
Switch clean-local to CLEANFILES
More autotools cleanup.

Remove the ola/common.h plumbing, which was added back in the LLA days to try
and support a thread safe client library.
Merge branch 'master' of https://github.com/OpenLightingProject/ola into slotinfo