Bump CMake version to 3.18

[typesanitizer (Varun Gandhi)]

@compnerd

Another option would be to actually have two separate versions of CMake and use that specifically for the specific target, but of course that incurs a complexity in the build system.

I'm not suggesting that any one solution is better, just trying to make sure that all the information is available so we can make an informed decision.

Given that how many different failure modes people hit in practice with the build, IMO having a single CMake version is the way to go; if something goes wrong, debugging will be easier with one CMake rather than two.