Pipewire doesn't export its version defines, so the API checks added
in commit f0c6c4eb1f effectively disable screencasting, whoops.
Breaking changes like this should disappear once the library stabilizes,
so simply define the version ourselves instead of writing a "proper"
test with AC_COMPILE_IFELSE() ...
https://bugzilla.gnome.org/show_bug.cgi?id=788572