Visual Studio Builds: Update README.txt's

Let people know that for Visual Studio builds GLib is a hard requirement,
and let the people also know that SDL-2.x is now used for builds supporting
the SDL winsys.  Also let people know that current situation regarding
building and linking Cogl applications with the SDL winsys built in.
Visual Studio Builds: Update .def Files Generation

Define COGL_HAS_GTYPE_SUPPORT during the preprocessing of the .symbols
files so that the _get_gtype symbols can be exported.