build: run-ci makefile rule
Adding a single rule to be called from Jenkins.
Jenkins jobs typically call: python ./configure make -j $(getconf _NPROCESSORS_ONLN) make test-ci
After this change, we can have Jenkins call: make run-ci -j $(getconf _NPROCESSORS_ONLN)
This allows us to customize how we call configure for different repos or branches (e.g. joyent\node).