CONTRIBUTING.md: mention coding style
Use $(MAKE) instead of ${MAKE}, and fixup two more make invocations.

Include a new SINGLE_MAKE which can be used to invoke make but using
only a single job, and in way that avoids any warnings from make.
Use SINGLE_MAKE to execute the tests, since they're meant to be run
serially.

Also, prefer the use of $(MAKE) to avoid invoking an extra subshell
(saves some time).
Clean-up main.c: follow STRNICMP replacement.
Be more explicit about OS X deps installation
Adding darwin platform to common.sh build script
README.md: remove lingering references to sha1sum in OSX instructions
Fixed nvim homebrew formula

I now also do make install
homebrew: deparallelize build