run stack setup when building with stack

This gets ghc installed if it's not already.

Motivation: Ævar's git test that runs git-annex test using git
can be called with BUILDER=stack, but without stack setup being run
sometime, the stack build will fail.

Makefile

@@ -18,6 +18,8 @@ tmp/configure-stamp: Build/TestConfig.hs Build/Configure.hs
 	if [ "$(BUILDER)" = ./Setup ]; then ghc --make Setup; fi
 	if [ "$(BUILDER)" != stack ]; then \
 		$(BUILDER) configure --ghc-options="$(shell Build/collect-ghc-options.sh)"; \
+	else \
+		$(BUILDER) setup; \
 	fi
 	mkdir -p tmp
 	touch tmp/configure-stamp