[OpenSmalltalk/opensmalltalk-vm] 80f639: Add MacOSX10.15.sdk to Makefile.flags [ci skip]

 
  Branch: refs/heads/Cog
  Home:   https://github.com/OpenSmalltalk/opensmalltalk-vm
  Commit: 80f639a99de57cb016bd3be5d6c53344a8f67ba5
      https://github.com/OpenSmalltalk/opensmalltalk-vm/commit/80f639a99de57cb016bd3be5d6c53344a8f67ba5
  Author: Fabio Niephaus <[hidden email]>
  Date:   2020-01-14 (Tue, 14 Jan 2020)

  Changed paths:
    M build.macos64x64/common/Makefile.flags

  Log Message:
  -----------
  Add MacOSX10.15.sdk to Makefile.flags [ci skip]

This is necessary, but not sufficient (see #470), to be able to build OSVM on macOS Catalina.