add third_party dir to git ignores
Move GoogleMock source directory to third_party
disable svn checkout of GoogleMock if the right revision is already checked out into
the source dir
fix sgvectors' refcount - to 0 -> ref -> 1 upon init
Merge pull request #1419 from votjakovr/develop

Add third_party dir to git ignores
Fix spinlock detection with clang
Add the right pthread library flag to the linking flags
Fix location of try_compile of spinlock test