Add a rule to allow us to sdist libraries easily

ghc.mk

@@ -1080,6 +1080,15 @@ SRC_CC_OPTS += -D__GLASGOW_HASKELL__=$(ProjectVersionInt)
 SRC_CC_OPTS += -I$(GHC_INCLUDE_DIR)
 endif
 
+# -----------------------------------------------------------------------------
+# sdisting libraries
+
+# Use manually, with e.g.:
+#     make sdist_directory
+
+sdist_%:
+	inplace/bin/ghc-cabal sdist libraries/$* dist-install
+
 # -----------------------------------------------------------------------------
 # Cleaning