Correct the info on G_PLATFORM_WIN32, now that we only define it when
GLib is built for native Windows, not Cygwin. Make it clear that it
should not be used to check for conditional compilation for Cygwin
builds of GLib.
This release series of GLib began using features that are provided in
the Windows 8 SDK and later for Visual Studio builds. This also means
that it is no longer possible to build GLib with Visual Studio 2008 nor
2010 since the Windows 8+ SDKs do not work with those compiler versions.
Mention that people that still need to use those Visual Studio versions
should continue sticking on to glib-2.66.x, and so remove the section
about the workarounds that need to be applied for Visual Studio 2008
builds, since they are no longer applicable.
