Set up CI to do split legacy build for macOS 10.9-10.12#1331

ychin merged 1 commit intomacvim-dev:mastermacvim-dev/macvim:masterfrom

ychin:split-ci-macos10.13-10.9-legacyychin/macvim:split-ci-macos10.13-10.9-legacyCopy head branch name to clipboard