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. While at it, use
pkg-config to get the library version, replacing our custom parsing code
which does not work.

This breaks the Applications dependency freeze, but it's an important
fix so it should go in.

1. ark master flatpak now can build the new libzip plugin.
2. ark now builds the libzip plugin only if libzip >= 1.2 is found.