Incorrect (too low) optimisation level for product builds

As part of MB-13722 & MB-13723, the production build type was changed from "Release" to "RelWithDebInfo", with a corresponding change to the C & C++ RelWithDebInfo flags to still build at the same optimization level.

Unfortunately the change was unsuccessful, as CMake requires the variables to be marked as CACHE for them to override the default (also CACHE) variable.