By default, only HsColour the docs if we find HsColour. Fixes trac #3004.

If you manually set HSCOLOUR_SRCS=YES then the build will fail if
HsColour wasn't found.

mk/config.mk.in

@@ -185,11 +185,6 @@ BootingFromUnregisterisedHc = @BootingFromUnregisterisedHc@
 #
 XMLDocWays=
 
-# Should we build haddock docs?
-HADDOCK_DOCS = YES
-# And HsColour the sources?
-HSCOLOUR_SRCS = YES
-
 # Should we build latex docs?
 LATEX_DOCS = NO
 
@@ -1192,6 +1187,15 @@ ALEX_VERSION		= @AlexVersion@
 #
 GHC_ALEX_OPTS		= -g
 
+# Should we build haddock docs?
+HADDOCK_DOCS = YES
+# And HsColour the sources?
+ifeq "$(HSCOLOUR)" ""
+HSCOLOUR_SRCS = NO
+else
+HSCOLOUR_SRCS = YES
+endif
+
 #
 # Options for compiling in different `ways'. 
 #