nut regressed cross building a while ago. The notable symptom is

| checking for Mozilla NSS version via pkg-config... none found

and that's surprsing given that it correctly detects a triplet-prefixed
pkg-config and also correctly depends on the host's nss development
package. A bit of investigation reveals that PKG_CONFIG_LIBDIR ends up
being set to /dev/null and from there one can locate
m4/nut_check_pkgconfig.m4 with relative ease as the source of culprit.
It checks whether the discovered $PKG_CONFIG is prefixed with $host
(which can be e.g. aarch64-unknown-linux-gnu) while the actualy used one
is prefixed with $host_alias (which can be e.g. aarch64-linux-gnu). It
concludes that pkg-config is unsuitable and messes with
PKG_CONFIG_LIBDIR as a result.

I'm attaching a patch that extends the patterns to cover other sensible
pkg-config values and that happens to already fix Debian cross buils.
Would you mind applying it and also forwarding it upstream?

Helmut