[GHC] #10157: HSCOLOUR_SRCS=YES fails mysteriously when no HsColour executable available

[GHC]

#10157: HSCOLOUR_SRCS=YES fails mysteriously when no HsColour executable available
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              Reporter:  ezyang      |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  low         |         Milestone:
             Component:  Build       |           Version:  7.11
  System                             |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 We seem to fill in the `--with-hscolour=` flag with an empty string, which
 causes a plain old build failure. Configure probably ought to fail if
 `HSCOLOUR_SRCS=YES` and there is no hscolours.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10157>
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