Allow Build of Graphene on Visual Studio

Hi,

This is the set of changes I had to build (and use) Graphene on Visual Studio, which I have tested with Visual Studio 2008 (the minimum MSVC version that is supported to build the GTK+/Clutter stack).  So far, this makes the code buildable on Visual studio and allows the test programs to pass (with the SSE initrinistics enabled for x64, and with the basic scalar instructions for x86), but does not yet contain the build files for Visual Studio builds.  I have checked that these patches do pass 'make distcheck' on my Fedora 20 system, but please note that the optimizations for SSE2 intrinistics aren't supported on 32-bit builds, like in the case of GCC < 4.2.  More investigation is needed for such optimizations on 32-bit builds, but it is probably possible to do that with the SSE2 initrinistics on Visual Studio 2013 and later using __vectorcall, but I need to check with that later.

With blessings, thank you!
we probably want to leave the GCC check in place, as `typeof()` is a GCC extension. Clang supports it in compatibility mode, but this would fail if we used SunCC or ICC.