contrib: qt: use system zlib

Rebased on !2655 (merged); only the last two commits are of interest.

Use system zlib and correct the QMake link flags.

Some questions:

• we could use -system-zlib to fail if the system zlib is not available. Is that preferable over the fallback to the vendored zlib?
• the link flags were needed because otherwise the Qt conftest fails to find the system version of zlib. But why isn't this handled with pkg-config? Why is pkg-config disabled for the whole of qt?