Fixed mesos-tidy build parallelism. (#427)
Fixed mesos-tidy build parallelism. (#427)

- Export `CMAKE_BUILD_PARALLEL_LEVEL`, otherwise it's not passed to
  child cmake processes.
- Do not pass `--parallel` to cmake processes: if `--parallel` is passed
  without an argument, cmake/make will interpret it by default as
  unbounded parallelism, despite `CMAKE_BUILD_PARALLEL_LEVEL` being set.

After this change, the build parallelism is correctly set to the number
of cores instead of unbounded.
dforsyth/mesos,dforsyth/mesos,dforsyth/mesos,dforsyth/mesos,dforsyth/mesos,dforsyth/mesos,dforsyth/mesos