This is a slight hack for adding an uninstall target. It parses the
install manifest and removes all of those same files. If your program
generates files at runtime, this uninstaller won't even know they exist
and will leave them. There are a few assumptions made here, such as that
your project has a folder called "cmake" that contains the
cmake_uninstall.cmake file. Credit is due to the original author but
I've had this one for so long, I can't remember the origins.