Need help diagnosing weird build error of mpv on Catalina

[Zhenfu Shi]

Hi guys, 
I am having trouble making mpv build on Catalina with MacPorts. There seems to be some sort of dependency conflict preventing the build. There are ways to avoid the error, but I can’t figure out how to add a (proper) fix in the portfile. The WIP PR is here: https://github.com/macports/macports-ports/pull/7080 , it contains more info.

Thanks in advance!