Add an OS-level semaphore module.
Bump version.
New 'cabal install' flag: '--max-linker-jobs'.
New 'setup build' flag: '--max-linker-jobs-semaphore'.
Split 'filterBuildFlags' and 'setNumJobs'.
New helper function: 'maybeToFlag'.
Actually implement the '--max-linker-jobs' option of 'cabal install'.
Actually implement the '--max-linker-jobs-semaphore' option of 'setup build'.
Make it more clear what is and what isn't a critical section.