Accept any pkg-config --modversion output

This results e.g. in

    cabal: The pkg-config package 'blas' version ==0.3 || >0.3 is required but the
    version installed on the system is version 0.2.20+ds

output instead of failing to parse 0.2.20+ds as a PkgconfigVersion.
We don't need to parse the `pkg-config --modversion` output
as strictly as the versions in `.cabal` file.
I.e. at all, as `PkgconfigVersion` comparison works somehow
on any possible version strings.