Install should check for for cmd-line dev tools on Mac OS

[ressy]

On a fresh install of Mac OS some of the developer tools needed when installing devtools aren't present by default, so we see a dialog box pop open prompting to install. The installer should handle this better instead of just proceeding and failing to install packages from that first attempt.

On the 10.15.1 system I have access to, the binary that handles the installation is:

/System/Library/CoreServices/Install Command Line Developer Tools.app/Contents/MacOS/Install Command Line Developer Tools

...so we could monitor that to see when it's finished. (The commands themselves that prompt for the install return immediately, so we can't wait on those.)

Maybe helpful:

http://osxdaily.com/2014/02/12/install-command-line-tools-mac-os-x/