Qt couldn't be configured with -system-zlib on macOS, because qmake
failed to find "/usr/lib/libz.dylib".
The library path is something along the lines of
"/Applications/Xcode.app/.../MacOSX10.14.sdk/usr/lib" which doesn't
contain "libz.dylib". But it contains "libz.tbd", which is a YAML-file
pointing to "/usr/lib/libz.dylib". One can pass the absolute path to
this tbd file to the linker, which will then pick up
"/usr/lib/libz.dylib".
Introduce a new variable QMAKE_EXTENSIONS_AUX_SHLIB, which is a list
of auxiliary extensions for shared libs, and add the "tbd" extension
to it on macOS.
Change-Id: I083b79a69d00232e35f9d6164ffa86cb473f1742
Fixes: QTBUG-72745
Fixes: QTBUG-72964
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
69bd238ad8