[cairo] Mutex support on win32

[Damian Frank]

I couldn't find any information for why Makefile.win32 has
-DCAIRO_NO_MUTEX=1 in it by default.  I've been routinely removing
that for my own MSVC-based builds, but lately I'm seeing some
corruption in a multithreaded test.  It's almost certainly my fault,
not cairo's, but I wanted to make sure that the cairo win32 mutex
implementation is, in fact, expected to work.  Can anyone confirm
this?

Thanks.

Damian