no way to reduce build parallelism.

[cg505]

My laptop becomes pretty much unusable during build if max parallelism is used - I want to be able to run the build in the background, so I set CMAKE_BUILD_PARALLEL_LEVEL to 1 (acknowledging that the build will take longer).
However, it seems build.rs unconditionally overrides CMAKE_BUILD_PARALLEL_LEVEL, so this change has no effect. It seems there is actually no way to control the build parallelism.
I think if CMAKE_BUILD_PARALLEL_LEVEL is already set in the environment, it should not be overridden.

[MarcusDunn]

a PR to this effect would be great.