mysqmail fails to cross build from source, because debian/rules does not
pass any cross tools to make. The easiest way of doing so is using
dh_auto_build. While this fixes the build, the use of the build
architecture strip is the next problem. The explicit stripping step has
other downsides. It also breaks generation of -dbgsym packages as well
as DEB_BUILD_OPTIONS=nostrip. It is best to rely on dh_strip alone. I'm
attaching a patch that fixes both problems for your convenience.

Helmut