FreeBSD: X needs Python 3.5 at least, but 2.7 was specified.

When trying to compile using the ports system, the error message that something needs Python 3.x, but 2.7 was specified is quite common.
This is due to the ports system "remembering" the python version that was specified by a top level package, while a dependency may require something else.

The solution is to manually compile the dependency first.

In my case I was trying to build sdl_sound, but it failed due to meson-0.51.2 needs Python 3.5 at least, but 2.7 was specified.
To fix this I compile meson first, then I go back and compile sdl_sound.