Pcap / CUPS versions not checked

[Salamandar]

As of today, dependency('cups|pcap', version: ">=10000") does not fail.
The reason is that version_compare is called inside PkgConfigDependency.__init__(), but not inside {Cups,Pcap}Dependency.__init__().

Maybe a better solution would be to add a check_version(self) to the ExternalDependency that would do the job without having to repeat the same code in each specialized dependency.

[Salamandar]

PR #2555 fixes that.

[nirbheek]

Fixed with #2512.