Make sparse requirement optional

Whilst I totally understand the desire to run sparse over the code when compiling during development to catch problems, this is an extra burden on packagers of the code because it means another build dependency and the potential for errors when sparse or the kernel code changes.

See for example, all the pieces of software that use -Werror. Sounds like a good idea until the compiler or glibc or some other dependency changes, and causes a new warning which then breaks the build. Fatal error checking should be optional, so that distribution packagers can disable it.

Specifically, I'm referring to the C=2 arguments in the CMakeLists.txt.