Since 35200a29ea we build complete installers using CMake, meaning that there's no need for BuildUtil anymore.
MSBuild projects that are not migrated to CMake yet are kept for reference.
This commit also updates BUILD_WINDOWS.md so that it mentions Visual Studio 2019 instead of 2017.
When loading the DLL file by the LoadLibrary() function in Windows VPN programs, we changed the behavior not to search the current directory. Based on this improvement, even if there are untrusted DLL files in the calendar directory, it is now safe to avoid the problem of unexpected security problem caused by the default loading behavior of Windows. Acknowledgments: This is based on a report by Herman Groeneveld, aka Sh4d0wman.
