[GHC] #9807: Clarify warning if LLVM is not found by configure

[GHC]

#9807: Clarify warning if LLVM is not found by configure
-------------------------------------+-------------------------------------
       Reporter:  ihmccreery         |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  low                |               Milestone:
      Component:  Build System       |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  Documentation bug
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 When running `./configure` to build GHC, and `opt` is not found, the
 current output is:

 > <no location info>:
 > Warning: Couldn't figure out LLVM version!
 > Make sure you have installed LLVM
 > ghc: could not execute: opt
 > failed to compile

 When I first saw this, I thought that this was causing the entire compiler
 build to fail.  To clarify that this isn't a critical error, should it be
 something more like:

 > checking for llvm... no

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9807>
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