Replacing SDL threads with pthread.

Replacing SDL threads with pthread.

For some reason I thought SDL thread handling would be much simpler... but
the migration to posix pthread went very smooth and painless. Less code
even, and I even notice a slight performance increase!

All threading code is still wrapped in blenlib/intern/threads.c
Only real change was making the callback functions to return void pointer,
instead of an int.

The mutex handling is also different... there's no test anymore if a
mutex was initialized, which is a bit confusing. But it appears to run
all fine still. :)

Nathan Letwory has been signalled already to provide the Windows pthread
library and make/scons linking. For MSVC we might need help from someone
else later though.