Building with '+universal' variant on MacOS 10.13.6

[RobbieNutland (Robbie Nutland)]

Just making a note of this should it be of benefit to anyone else.

The build and destroot phases of a 'gtk3 +universal' install fail on macos 10.13.6 as ninja complains it does not understand build targets: 'CC='${configure.cc} -arch ${arch} and CC_FOR_BUILD='${configure.cc} -arch ${arch}'.

Updating the gtk3 Portfile per the below resolves the issue. (For context, 'Portfile' has been updated and successfully builds, and 'Portfile backup' is the original file.

diff /opt/local/var/macports/sources/rsync.macports.org/macports/release/tarballs/ports/gnome/gtk3/Portfile /opt/local/var/macports/sources/rsync.macports.org/macports/release/tarballs/ports/gnome/gtk3/Portfile\ backup
91,92c91,94
<         lappend merger_build_env(${arch}) "CC=${configure.cc} -arch ${arch}" "CC_FOR_BUILD=${configure.cc} -arch ${arch}" "LDFLAGS=-L/opt/local/lib -arch ${arch}"
<         lappend merger_destroot_env(${arch}) "CC=${configure.cc} -arch ${arch}" "CC_FOR_BUILD=${configure.cc} -arch ${arch}" "LDFLAGS=-L/opt/local/lib -arch ${arch}"
---
>         lappend merger_build_args(${arch})  CC='${configure.cc} -arch ${arch}'
>         lappend merger_build_args(${arch})  CC_FOR_BUILD='${configure.cc} -arch ${arch}'
>         lappend merger_destroot_args(${arch})  CC='${configure.cc} -arch ${arch}'
>         lappend merger_destroot_args(${arch})  CC_FOR_BUILD='${configure.cc} -arch ${arch}'
301d302
< 

Much like the ticket I also recently raised for 'groff', this is a selfish change just to aid those that may be attempting an install from a similar macos environment. I cannot confirm how this would impact other machines such as more recent arm-based macs.

Happy to provide more info upon request :)