cmake: Add a flag to enable LTO
This adds -DLLVM_ENABLE_LTO, rather than forcing people to manually
add -flto to the various _FLAGS variables.
Could we support a string flag instead of a boolean? (I don't know if CMake options can be enums)
The clang "-flto" accepts both "full" and "thin" as an argument, and it would be nice to support it first class in the configuration.