Add pthread configuration similar to autoconf

The new OmeroCpp/PThread SConscript file is now available
for checking various pthread configurations similar to
autoconf.

The file is called once by SConstruct and then recursively
calls itself in order to test the various pthread flags.
This is mostly determined by some strange calling errors in
SCons, but should allow with a little more work a fairly
self-contained pthread tool.

Note: several sections are still commented out since they
are unlikely to be immediately important for OMERO.