[MacPorts] #62403: libvpx build does not obey deployment target setting

[MacPorts]

#62403: libvpx build does not obey deployment target setting
--------------------+--------------------
 Reporter:  kencu   |      Owner:  (none)
     Type:  defect  |     Status:  new
 Priority:  Normal  |  Milestone:
Component:  ports   |    Version:
 Keywords:          |       Port:  libvpx
--------------------+--------------------
 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 tsrget setting
 instead of using os.major.

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Ticket URL: <https://trac.macports.org/ticket/62403>
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