[GTK3] Zlib required when broadway is enabled.
When building with -Dbroadway_backend=true
GTK+ 3 requires to link against Zlib but does so by specifying -lz
in the GDK pkg-config file instead of it being in the pkg-config's file Requires
field. This causes some problems in package managers like GNU Guix or Nix as it causes packages that depend on GTK to list Zlib as a dependency directly instead of relying on pkg-config mechanisms to find dependencies.
See GNU Guix bug with attached patches to work around the issue: