make makefile work for darwin(mac)

Makefile

@@ -4,6 +4,7 @@ outdoc_files = $(addprefix out/,$(doc_sources:.md=.html))
 docassets = $(addprefix out/,$(wildcard doc/assets/*))
 
 VERSION = $(shell node -e "console.log( require('./src/package.json').version )") 
+UNAME := $(shell uname -s)
 
 docs: $(outdoc_files) $(docassets)
 
@@ -14,7 +15,10 @@ out/doc/assets/%: doc/assets/%
 out/doc/%.html: doc/%.md
 	mkdir -p $(@D)
 	node tools/doc/generate.js --format=html --template=doc/template.html $< > $@
-	sed -i 's/__VERSION__/${VERSION}/' $@
+	ifeq ($(UNAME),Darwin)
+		sed -i '' 's/__VERSION__/${VERSION}/' $@
+	else
+		sed -i 's/__VERSION__/${VERSION}/' $@
 
 clean:
 	rm -rf out/