Fix libraries/index.html's haddock dependency on Windows

ghc.mk

@@ -703,7 +703,7 @@ $(foreach p,$(STAGE0_PACKAGES),$(eval libraries/$p_dist-boot_DO_HADDOCK = NO))
 
 # Build the Haddock contents and index
 ifeq "$(HADDOCK_DOCS)" "YES"
-libraries/index.html: inplace/bin/haddock $(ALL_HADDOCK_FILES)
+libraries/index.html: inplace/bin/haddock$(exeext) $(ALL_HADDOCK_FILES)
 	cd libraries && sh gen_contents_index --inplace
 ifeq "$(phase)" ""
 $(eval $(call all-target,library_doc_index,libraries/index.html))