Don't fail validate due to warnings in Cabal

Duncan says that Cabal isn't guaranteed to be warning-free.

mk/validate-settings.mk

@@ -54,6 +54,11 @@ endif
 ######################################################################
 # Disable some warnings in packages we use
 
+# Cabal doesn't promise to be warning-free
+utils/ghc-cabal_dist_EXTRA_HC_OPTS += -w
+libraries/Cabal/cabal_dist-boot_EXTRA_HC_OPTS += -w
+libraries/Cabal/cabal_dist-install_EXTRA_HC_OPTS += -w
+
 # Temporarily turn off incomplete-pattern warnings for containers
 libraries/containers_dist-install_EXTRA_HC_OPTS += -fno-warn-incomplete-patterns
 

utils/ghc-cabal/ghc.mk

@@ -34,7 +34,8 @@ $(GHC_CABAL_DIR)/dist/build/tmp/ghc-cabal$(exeext): $(GHC_CABAL_DIR)/Main.hs | $
 	       -hidir bootstrapping \
 	       -ilibraries/Cabal/cabal \
 	       -ilibraries/filepath \
-	       -ilibraries/hpc
+	       -ilibraries/hpc \
+	       $(utils/ghc-cabal_dist_EXTRA_HC_OPTS)
 	touch $@
 
 # touch is required, because otherwise if mkdirhier is newer, we