Is your feature request related to a problem? Please describe.
mpg123 supports ARM architectures; however, the portfile, automatically fails builds on either ARM architectures or UWP. ARM Windows support was added with #7934 so this might be a slight oversight. It seems that the intention is to fail builds on arm-uwp triplets, but vcpkg_fail_port_install operates on the disjunction of its parameters and not the conjunction.

Another issue is with cross-compilation; vcpkg ports that build with CMake can be cross-compiled easily by writing an appropriate triplet for the toolchain and setting CC and CXX environment variables. However, mpg123 builds with autoconf, so the cross-compiler prefix must be passed to ./configure via the --host parameter.

Proposed solution
I customized the portfile locally and this works specifically to cross-compile to arm64. Modified portfile and corresponding triplet. However, I think it would be appropriate to genericize this approach so it can be used for other compiler settings without locally editing the portfile, but I'm not familiar enough with the vcpkg approach to proceed further, so I'm looking for suggestions and pointers.