Override --jobs option value with an env var in build.rs
Override --jobs option value with an env var in build.rs

This patch enables build.rs to override --jobs option value of
make command with MAKE_LIBSASS_JOBS environment variable.

In typical containerization system (e.g. Docker), the number of CPU cores
which is visible from a process jailed into a container can be greater than
the container limitation.

This change is useful in the above situation. (Of course, not only that!)

compass-rs/sass-rs