It seems reasonable to have long options in MAKEOPTS, like e.g. MAKEOPTS="--jobs=8 --load-average=8". These are obviously correctly passed through to make, but not converted to the respective -j and -l options for ninja. Seems to me that cmake.eclass and/or multiprocessing.eclass should be responsible for this...? One could argue that this should be solved by the user, but I'd counter that the user should not have to worry about gmake vs. ninja peculiarities, especially if the appropriate quirks can be derived unambiguously from the user input.
*** This bug has been marked as a duplicate of bug 559064 ***