Don't error on missing Perl, just warn and disable object splitting.
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

sdiehl/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc