ghc-pkg dependency on ghc-cabal should be an order-only dependency,

otherwise we repeatedly try to rebuild ghc-pkg if ghc-cabal is newer.

utils/ghc-pkg/ghc.mk

@@ -44,9 +44,11 @@ endif
 
 endif
 
-# depend on ghc-cabal, otherwise we build Cabal twice when building in parallel
+# depend on ghc-cabal, otherwise we build Cabal twice when building in parallel.
+# (ghc-cabal is an order-only dependency, we don't need to rebuild ghc-pkg
+# if ghc-cabal is newer).
 # The binary package is not warning-clean, so we need a few -fno-warns here.
-utils/ghc-pkg/dist/build/$(utils/ghc-pkg_dist_PROG)$(exeext): utils/ghc-pkg/Main.hs utils/ghc-pkg/Version.hs $(GHC_CABAL_INPLACE) | bootstrapping/. $$(dir $$@)/.
+utils/ghc-pkg/dist/build/$(utils/ghc-pkg_dist_PROG)$(exeext): utils/ghc-pkg/Main.hs utils/ghc-pkg/Version.hs | bootstrapping/. $$(dir $$@)/. $(GHC_CABAL_INPLACE) 
 	"$(GHC)" $(SRC_HC_OPTS) --make utils/ghc-pkg/Main.hs -o $@ \
 	       -no-user-package-conf \
 	       -Wall -fno-warn-unused-imports \