Filter colons out of ghc-config-mk

Otherwise the colon confuses make on Windows

testsuite/mk/boilerplate.mk

@@ -143,8 +143,10 @@ $(TOP)/mk/ghc-config : $(TOP)/mk/ghc-config.hs
 
 empty=
 space=$(empty) $(empty)
-ghc-config-mk = $(TOP)/mk/ghcconfig$(subst $(space),_,$(subst /,_,$(subst \,_,$(TEST_HC)))).mk
+ghc-config-mk = $(TOP)/mk/ghcconfig$(subst $(space),_,$(subst :,_,$(subst /,_,$(subst \,_,$(TEST_HC))))).mk
 
+$(info TOP is $(TOP))
+$(info ghc-config-mk is $(ghc-config-mk))
 $(ghc-config-mk) : $(TOP)/mk/ghc-config
 	$(TOP)/mk/ghc-config "$(TEST_HC)" >"$@" || $(RM) "$@"