Makefile needs to improve treatment of HsColour

Fresh checkout, a few new patches, sh boot & configure are OK, but 'make' results in

: nr@labrador 10435 ; make
HsColour needed but wasn't found.
Set HSCOLOUR_SRCS=NO if you don't want to use it
exit 1
make: *** [check-tools] Error 1

Things that are wrong here:

1. 'sh boot; ./configure; make' failed. This should never happen.
2. I'm not told where to set HSCOLOUR_SRCS=NO.
3. The missing software should be detected by ./configure and the variable set then.
4. The error message (which should be a warning message from configure) does not tell me how to get and install the HsColour software. The error message should include information along the following lines:

If you do want to use the software, the source is at

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hscolour

If you've got cabal-install then:

$ cabal update
$ cabal install hscolour

1. Finally, the message should say what HsColour is so that the rank amateur (me) can figure out if it's worth installing.

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
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