* deps: Use CMAKE_BUILD_PARALLEL_LEVEL for dependency builds.
If set when initial CMake is run, don't hard-code the number of CPUs for all the
recursive child builds.
* build_linux: Pass -j1 for dependency build.
This is recommended by the comment at the top of deps/CMakeLists.txt, and makes
sense as the recursive build steps will pick up
CMAKE_BUILD_PARALLEL_LEVEL (otherwise the number of parallel jobs is squared).
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