Draft: meson.pm: Use meson env2mfile and pass a --native-file

Meson prefers to describe its build environment using cross/native files that record all relevant configuration and paths. The debcrossgen tool is now deprecated in favour of "meson env2mfile" and the new tool supports producing a native file. These files not only capture tool paths but also compiler flags. As such, we no longer pass such flags via the environment.