Darwin: Detect broken NM program at configure time
Darwin: Detect broken NM program at configure time

Some recent versions of XCode ship a broken version of `nm`. Detect this
at configure time, and error out with a suggestion to rerun configure
with a `--with-nm=...` argument.

Test Plan: Test of Linux and OS X

Reviewers: carter, hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2425

olsner/ghc,mettekou/ghc,mettekou/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,snoyberg/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,olsner/ghc,snoyberg/ghc,ezyang/ghc,sdiehl/ghc,snoyberg/ghc,mettekou/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,mettekou/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,snoyberg/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,mettekou/ghc,sdiehl/ghc,mettekou/ghc,mettekou/ghc,shlevy/ghc,snoyberg/ghc,olsner/ghc,olsner/ghc,sdiehl/ghc,snoyberg/ghc,snoyberg/ghc,olsner/ghc,olsner/ghc