Allow to pass extra flags to make deps.

I've seen somewhere here that it is recommended to build Julia in single thread, like ```make -j1```.
But it means that dependencies will also be built in single thread. With this change, I hope to speed up compilation of dependencies on multi-cpu systems, by setting (according to cpu count) ```MAKE_DEPS_OPTFLAGS = -j4``` in ```Make.user``` file. Will that work?