avidemux tries to build partially for x86_64 on Apple Silicon

[ryandesign (Ryan Carsten Schmidt)]

On an Apple Slicon Mac, avidemux tries to build parts of itself for x86_64 despite not being asked to do so, and it fails:

​https://build.macports.org/builders/ports-11_arm64-builder/builds/32282/steps/install-port/logs/stdio

ld: warning: ignoring file ../../ffmpeg/build/libavutil/libADM6avutil.56.dylib, building for macOS-x86_64 but attempting to link with file built for macOS-arm64
Undefined symbols for architecture x86_64:
  "_av_set_cpu_flags_mask", referenced from:
      CpuCaps::setMask(unsigned int) in ADM_cpuCap.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)