Ninja Multi-Config: Make cross-config building opt-in

Many users will want to use the Ninja Multi-Config generator like a traditional Visual Studio-style multi-config generator, which doesn't mix configurations - custom commands are built using target executables of the same configuration the command is for. We do not want to force these people to generate an N*N build matrix when they only need N*1, especially if they have lots of targets. Add a new variable, CMAKE_NINJA_CROSS_CONFIG_ENABLE, to opt-in to the cross-config build matrix.