Don't error on missing Perl, just warn and disable object splitting.
Summary: If Perl isn't needed, we don't need to error out. Since all Perl is used for is the splitter, we can just warn. Test Plan: GHC CI Reviewers: bgamari, hvr, austin Reviewed By: bgamari Subscribers: thomie, erikd Differential Revision: https://phabricator.haskell.org/D2986 GHC Trac Issues: #13141
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