libvpx build does not obey deployment target setting

[kencu (Ken)]

we build libvpx forcing the target like this:

force-target=${my_arch}-${os.platform}${os.major}-gcc

this builds against the build machine's os version.

when a user tries to build libvpx against an older deployment target, this build does not obey that deployment target.

​https://lists.macports.org/pipermail/macports-users/2021-March/049617.html

perhaps our build target should incorporate the deployment target setting instead of using os.major.