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

[GHC]

#10157: HSCOLOUR_SRCS=YES fails mysteriously when no HsColour executable available
-------------------------------------+-------------------------------------
        Reporter:  ezyang            |                   Owner:
            Type:  bug               |                  Status:  patch
        Priority:  low               |               Milestone:  7.12.1
       Component:  Build System      |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D1232
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Comment (by Thomas Miedema <thomasmiedema@…>):

 In [changeset:"1b8eca18fc2bb9ccb4fd3246ac48318975574722/ghc"
 1b8eca18/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="1b8eca18fc2bb9ccb4fd3246ac48318975574722"
 Build system: check for inconsistent settings (#10157)

 `configure` currently detects when the docbook and hscolour tools aren't
 available, and instead of failing outright (as it does for missing alex
 and happy), sets some variables in mk/config.mk to tell `make` not to
 build the documentation.

 Sometimes, however, you want to really make sure all documentation gets
 built, fully colourized. For example when making a release. To do so,
 you can override the mentioned variables from mk/config.mk in
 mk/build.mk (e.g. set HSCOLOUR_SRCS=YES).

 This patch adds some error checking to make sure that doing so will not
 result in weird build failures when those tools are still missing.

 Test Plan: ran `make` a couple of times, with different mk/config.mk
 settings.

 Reviewed by: austin

 Differential Revision: https://phabricator.haskell.org/D1232
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10157#comment:2>
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