Invalid ar syntax in cpp port: `ar: --: No such file or directory`

Fails to build on macOS:

make: Entering directory `/opt/local/var/macports/build/_opt_CatalinaPorts_graphics_qrcodegen-cpp/qrcodegen-cpp/work/QR-Code-generator-2c9044de6b049ca25cb3cd1649ed7e27aa055138/cpp'
mkdir -p .deps
touch .deps/timestamp
/opt/local/var/macports/build/_opt_CatalinaPorts_graphics_qrcodegen-cpp/qrcodegen-cpp/work/compwrap/cxx/usr/bin/clang++ -Os -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64 -stdlib=libc++ -std=c++11 -O -c -o qrcodegen.o -MMD -MF .deps/qrcodegen.d qrcodegen.cpp
/opt/local/var/macports/build/_opt_CatalinaPorts_graphics_qrcodegen-cpp/qrcodegen-cpp/work/compwrap/cxx/usr/bin/clang++ -Os -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64 -stdlib=libc++ -std=c++11 -O -c -o QrCodeGeneratorDemo.o -MMD -MF .deps/QrCodeGeneratorDemo.d QrCodeGeneratorDemo.cpp
ar -crs libqrcodegencpp.a -- qrcodegen.o
ar: --: No such file or directory
make: *** [libqrcodegencpp.a] Error 1

Same error with 1.8.0 release and 2c9044de6b049ca25cb3cd1649ed7e27aa055138 master.

Start from the C++ build invoked by make and the command that produces libqrcodegencpp.a. Reproduce the archive step on macOS, then inspect how its object-file arguments are assembled. Done means the C++ build completes and the static library is created without treating -- as an input file.