'make everything' tweaks

First commit: only build `matcaffe` if `MATLAB_DIR` is set.

Second commit ("optional"): don't do `runtest` as part of everything.  This is totally subjective, but it's more useful to me to have a target that just "builds" everything (and/or reassure myself that I have built everything with a poetic `Nothing to be done for everything`).  Running the whole test suite takes a while, so for the few times I want to do that I'd rather just spend 8 extra keystrokes to do `make everything runtest`.  @longjon (who wrote the `everything` build target), what do you think? If you like the `runtest` I can just add a new target to only do the build.