zlib: clean up static/shared distincion

This is kind of a mess, but basically:

- static=true, shared=true means to build statically but move it to
  the static output
- static=true, shared=false means to build statically and leave it in
  the main output
- static=false, shared=true means to not build static at all

Confusingly, the old default was static=true, shared=true even though
static=false? Still can’t figure out what was meant by that.