A few technical notes, because this was harder than it needed to be due to development environment cruft.
- Remove MSYS from your PATH.
- Disable Windows Subsystem for Linux.
- Remove old versions of GCC from your PATH.
- Install MinGW-64 using WinBuilds.
a. The package URL was incorrect. If needed, override it to http://win-builds.org/1.5.0/packages/windows_64/ .
winbuilds\bin. This may not be strictly necessary, but I'm disinclined to experiment at this point.
- Open CMake. Point to the source code and build target.
- Configure. Will succeed if paths are set properly.
- Set installation path.
- Open Terminal in build target directory (reminder: Shift+RMB).