Debian Stretch 9.4 libssl1.0.0 build error

Hit this just now while running buildprep on RPI Zero W, all packages current, nearly bare OS install.

The current libssl is v1.10, but libssl1.0.2 is also installed in base apparently.

This generated no ill-effects - buildprep proceeded, and waf build etc all ran fine.

snippet: `Processing triggers for libc-bin (2.24-11+deb9u3) ... Reading package lists... Done Building dependency tree
Reading state information... Done Package libssl1.0.0 is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source

E: Package 'libssl1.0.0' has no installation candidate Reading package lists... Done Building dependency tree`

full buildprep output attached.buildprep-out.txt