Ensure that the /DYNAMICBASE linker option (which is actually the
default) is enabled for all of our Visual Studio project builds, as the
gtk3-demo projects inadvertly disabled it.

Also, for x64 builds on MSVC 2012 or later, enable /HIGHENTROPYVA to
enhance the security of our binaries as well.

Pointed out by Ignacio Casal Quinteiro.