core-deps, world: Make mm-common a build-dependency
mm-common is listed as a dependency of core-deps/libsigcplusplus-2.0 and
world/libsigcplusplus, but it's needed only at build-time. It can be a
build-dependency of libsigcplusplus-2.0, libsigcplusplus and some other modules.
See #351 (closed)