configure: Catch case where LLVM tools can't be found
configure: Catch case where LLVM tools can't be found

Previously we didn't specify the prog-not-found value passed to
AC_CHECK_TOOLS.  Reported by @snowleopard in
https://github.com/snowleopard/hadrian/issues/415.

Test Plan: validate

Reviewers: austin, hvr

Subscribers: rwbarton, thomie, snowleopard, erikd

Differential Revision: https://phabricator.haskell.org/D3992

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