kmenc15 requires now mplayer to be installed. However on amd64 boxes it's often common to have mplayer-bin installed (due to the fact that not all codecs can run natively in 64bit mode - for example closed source drivers which are nom 32bit-only) it also provides mencoder and whole encoding capabilities, so my offer is to make kmenc15 dependent on mplayer or mplayer-bin and not mplayer only.
You can't compile anything against mplayer-bin.