Log Message:
rust: speed-up building; clean-ups

- use 'build' target for building, not 'dist'
- set jobs also for install target
- do not generate tarballs; we don't need them, but they take a lot of disk-space
- do not install 'src'
- do not generate 'install.log' nor 'uninstall.sh'
- on Darwin, use headerpad_max_install_names to be able to fix all dylibs
- make optimized bootstrap
- pkglint fixes
- get ready to depend on lang/llvm and devel/jemalloc