* Silence some warnings about things that MSVC 2015 warns about that
MSVC 2013 didn't.
* Change the solution so that it opens in Visual Studio 2015 if Visual
Studio 2015 is installed.
* Change ToolsVersion to 14.0 in the project files so that Visual
Studio 2015 does not put a warning in the build log about the version
being 12.0. This means that Visual Studio 2013 will have a warning,
but the warning is harmless, and if we have to choose, it's better
for VS 2013 to warn than VS 2015 to warn, in the long term. See
https://github.com/Microsoft/msbuild/issues/52#issuecomment-88674094.
* Build with the VS 2013 XP-compatible toolchain if using Visual Studio
2013 (including its msbuild); Otherwise build with the Visual Studio
2015 XP-compatible toolchain. See
https://social.msdn.microsoft.com/Forums/vstudio/en-US/ \
d06c3741-c637-4627-9b1a-1e068803a067/ \
setting-platformtoolset-value-based-on-vs-version.
3685ef6d97