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.

P.S. the mail.log is too big to attach. Is this file also helpful? Then I'll try to strip some lines off the middle.