support system dynamic libs for ffi

This adds support for system supplied dynamic libraries.  previously only share install was supported.

It uses `FFI::CheckLib` instead of doing the work itself, since this detection is useful in non alien contexts as well.

The prereq is relatively "light", as it has no non-core deps as far back as Perl 5.6.2 and no failures on the test matrix:

http://matrix.cpantesters.org/?dist=FFI-CheckLib+0.03

actual testing will be done via `Acme::Ford::Prefect::FFI`.