Add 'flow' checkout to Makefile (unused now)

Flow could be used to check Ecmascript test cases.

.gitignore

@@ -33,3 +33,4 @@ sax-js
 xmldoc
 FlameGraph
 dtrace4linux
+flow

Makefile

@@ -316,6 +316,7 @@ cleanall: clean
 	@rm -rf xmldoc
 	@rm -rf FlameGraph
 	@rm -rf dtrace4linux
+	@rm -rf flow
 	@rm -rf 595a36b252ee97110724e6fa89fc92c9aa9a206a.zip
 
 libduktape.so.1.0.0: dist
@@ -765,6 +766,10 @@ dtrace4linux:
 	# http://crtags.blogspot.fi/
 	$(GIT) clone --depth 1 https://github.com/dtrace4linux/linux.git dtrace4linux
 
+flow:
+	# https://github.com/facebook/flow
+	$(GIT) clone --depth 1 https://github.com/facebook/flow.git
+
 .PHONY: gccpredefs
 gccpredefs:
 	gcc -dM -E - < /dev/null