Testing the branch using data from a git submodule.

• Does the Portfile run git submodule update --init for us?
• We need to run ninja gimp-data/images/logo/gimp.icns manually in the end to build the ICNS application icon file for GIMP on macOS. Where does that run?

@Jehan I created this MR so that I could put my build results here. Feel free to close or whatever with it.