libgcc8 @8.2.0.1 fails to build on Mojave with "C compiler cannot create executables"

[p-vitt (pvitt)]

I just updated my OS to 10.14 Mojave and followed the Migration process described in the wiki. Three Mac Minis succeeded, but the first MacBook somehow failed:

Error: Failed to build libgcc8: command execution failed
Error: See /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_gcc8/libgcc8/main.log for details.

In main.log it says:

:info:build checking for C compiler default output file name...-
:info:build configure: error: in `/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_tarballs_ports_lang_gcc8/libgcc8/work/build/x86_64-apple-darwin18/libgomp':
:info:build configure: error: C compiler cannot create executables
:info:build See `config.log' for more details.
:info:build make[2]: *** [configure-stage1-target-libgomp] Error 77

In libgomp's config.log, it says error: unrecognized command line option '-V' or error: unrecognized command line option '-qversion'; did you mean '--version' or error: C compiler cannot create executables.

I reinstalled XCode, accepted the license, checked the Command Line Tools, everything is fine. I also tried a binary install, without success. port install -b libgcc8 fails with 404 on all probed sources.

Then I tried to install gcc7, but this also depends on libgcc8. ​Uninstalling libunwind doesn't help. I also had a look at some ​older ​tickets, without success.

Right now, I have no clue how to proceed. Any help appreciated.