libzip installs zipconf.h in $prefix/$libdir/libzip/include/. Some
systems (e.g. archlinux) symlinks this file under $prefix/$libdir/include
for compatibility reasons, but cmake fails on systems that do not (e.g.
flatpak). Since libzip supports pkg-config, we can just query pkg-config
which gives us the proper path we are looking for.
This breaks the Applications dependency freeze, but it's an important
fix so it should go in.