$ port provides /opt/local/lib/libunwind.dylib
/opt/local/lib/libunwind.dylib is provided by: libunwind
I am not sure what libunwind is. After encountering the sqlite3 issue I updated my ports a got the message
Error: libgcc-devel cannot be built while libunwind-headers is active.
Error: Please forcibly deactivate libunwind-headers, e.g. by running:
Error:
Error: sudo port -f deactivate libunwind-headers
Error:
Error: Then try again. You can reactivate libunwind-headers again later.
Possible multiple ports provide the same thing? I inactivated libunwind-headers, upgraded libgcc-devel, that activated libunwind-headers. That seemed to work. So they might be related to gcc?
$ port rdependents libunwind
The following ports are dependent on libunwind:
PDAL
grass7
qgis3
I suspect "configure script opportunistically finds it" is true.