Normally you should submit a main.log file when you encounter a build failure. But I tried a build on our Apple Silicon build machine and got the same problem, so that log is sufficient:
https://build.macports.org/builders/ports-11_arm64-builder/builds/20536/steps/install-port/logs/stdio
Something is supposed to be defining DarwinMachineDefines to the set of flags that are appropriate for your system. That "something" was probably written before Apple Silicon machines existed, so it doesn't define DarwinMachineDefines to anything on your system, so the literal string DarwinMachineDefines gets inserted instead, which is wrong.
I haven't yet found what is (supposed to be) defining DarwinMachineDefines in this case. I did find it for example here:
https://cgit.freedesktop.org/xorg/util/cf/tree/darwin.cf
And I found a previous reference to this kind of problem in #22528 (which was on Snow Leopard which was when Apple's gcc started defaulting to 64-bit compiles, so at that time it was probably the x86_64 architecture that wasn't being handled in the code.)
