Automatically handle Xcode CLT install if needed on MacOS

[zbeekman]

See, for example how the homebrew installer handles this.

I have written a function to test if we need to install the CLT, but am wondering at what point do we call it and do the installation. I'm guessing every single one of our dependencies probably needs the Xcode CLT on MacOS, so, I guess we put it near the beginning of install.sh? Here is a function to detect if CLT are missing:

install_xcodeCLT () {
    if [[ "${OSTYPE}" != [Dd][Aa][Rr][Ww][Ii][Nn]* ]]; then
        echo "false"
    else
        local xcode_dir
        xcode_dir="$(/usr/bin/xcode-select -print-path 2>/dev/null)"
        if [[ -z "${xcode_dir}" ]] || ! [[ -x "${xcode_dir}/usr/bin/make" ]] ; then
            echo "true"
        else
            echo "false"
	fi
    fi
}

Works safely on other systems.

Here is the clever magic that homebrew does to get CLT installed and the EULA signed: (from https://github.com/Homebrew/install/blob/master/install#L259-L285)

if should_install_command_line_tools?
  ohai "Searching online for the Command Line Tools"
  # This temporary file prompts the 'softwareupdate' utility to list the Command Line Tools
  clt_placeholder = "/tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress"
  sudo "/usr/bin/touch", clt_placeholder
  clt_label = `softwareupdate -l | grep -B 1 -E "Command Line (Developer|Tools)" | awk -F"*" '/^ +\\*/ {print $2}' | sed 's/^ *//' | tail -n1`.chomp
  ohai "Installing #{clt_label}"
  sudo "/usr/sbin/softwareupdate", "-i", clt_label
  sudo "/bin/rm", "-f", clt_placeholder
  sudo "/usr/bin/xcode-select", "--switch", "/Library/Developer/CommandLineTools"
end

# Headless install may have failed, so fallback to original 'xcode-select' method
if should_install_command_line_tools? && STDIN.tty?
  ohai "Installing the Command Line Tools (expect a GUI popup):"
  sudo "/usr/bin/xcode-select", "--install"
  puts "Press any key when the installation has completed."
  getc
  sudo "/usr/bin/xcode-select", "--switch", "/Library/Developer/CommandLineTools"
end

abort <<-EOABORT if `/usr/bin/xcrun clang 2>&1` =~ /license/ && !$?.success?
You have not agreed to the Xcode license.
Before running the installer again please agree to the license by opening
Xcode.app or running:
    sudo xcodebuild -license
EOABORT

My ruby, while poor, is good enough to translate that to bash, I believe... or come up with our own version.

[rouson]

@zbeekman Possibly we can knock this out relatively quickly in a pair programming session or mark it as "won't fix" and close it. I'll bel glad to help with modifying the installation scripts if we decide to fix this. Thoughts?