glycin fails to cross build from source. The error is:

| ../meson.build:1:0: ERROR: 'rust' compiler binary not defined in cross file [binaries] section

Evidently, the crossfile passed to meson lacks a rust option in the
binaries section. Whilst this is currently handled by debcrossgen, the
goal is to produce it using env2mfile, but env2mfile also does not
produce this option. Hence, I am attaching a patch for env2mfile to add
it. I am presently unable to test the patch, because glycin is now
bd-uninstallable again, but the resulting crossfile looks sensible.

Helmut