[cmake-developers] review request: fix-protobuf-threads

[Matthew Woehlke]

After chatting with Marcus how to resolve ParaView link errors due to 
things using Google Protobuf needing to link to pthread, I have updated 
FindProtobuf.cmake to also find the pthread library on UNIX platforms 
and include it in PROTOBUF_LIBRARIES.

This should fix link errors in a number of projects and/or remove the 
need for downstream users to add pthread themselves when using protobuf.

The branch (name in summary) is on stage. Please have a look if this is 
reasonable and can be merged to master once 2.8.11 is out.

Thanks,

-- 
Matthew