Darwin configure passes invalid flag -force_cpusubtype_ALL

Flag -force_cpusubtype_ALL is passed by configure (see https://gitlab.xiph.org/xiph/vorbis/-/blob/master/configure.ac#L205) unconditionally. This darwin flag was was only ever valid for PowerPC, and ignored for Intel and now Apple Silicon (ARM). In recent versions, the linker now flatly refuses is (in Xcode 15):

ld: unknown options: -force_cpusubtype_ALL

It should either be selectively passed for PowerPC or, at this stage, removed entirely.