Let's follow this rule: if build type is Debug or RelWithDebInfo, then be helpful and enable debug fission automatically so that we have optimal defaults for developers. But if build type is anything else, then the user must be adding -g manually for debug fission to be useful. If user is manually editing compiler flags, trust that the user knows best and will add -gsplit-dwarf is desired to avoid breaking distro debuginfo systems.

You can also use -DDEBUG_FISSION=ON/OFF to override WebKit's defaults if this guesses wrong, and I almost went with this solution downstream, but (a) this will probably confuse lots of distros or result in broken debuginfo generation, and (b) it's also just nicer to simplify the CMake file.