Allow opt-out of building managed (C++/CLI) projects

[jng]

I was experimenting with building MapGuide/FDO using the new "build tools" SKU of Visual C++ 2015

​https://blogs.msdn.microsoft.com/vcblog/2015/11/02/announcing-visual-c-build-tools-2015-standalone-c-tools-for-build-environments/

I was able to build most of the stack, but ran into a roadblock with the managed wrapper projects as the build tools SKU appears to lack the key tools/libs needed for compiling/linking C++/CLI code

In the context of building MapGuide, one does not need to have built managed .net FDO binaries anyways, so having some flag/configuration in the FDO build system that lets us skip these parts of the build would be great.