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This adds NMake Makefiles for Visual Studio 2013 and later, which will
replace the Visual Studio projects.  The main motivation for this is to
ease maintenace on the Visual Studio build files, especially as sources
are often added or removed, making the project files out-of-date as they
are not generated during 'dist make'.  This will share the various
filelist.am's so that any additions or removal of sources can be
reflected upon the Visual Studio builds immediately.

Note that as some code in the demos require compiler features that is
only available in Visual Studio 2015 or later, we only build the demos
if Visual Studio 2015 or later is used.