build: don't run find in non-existent directory

PR-URL: #97
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Makefile

@@ -79,7 +79,7 @@ uninstall:
 
 clean:
 	-rm -rf out/Makefile $(NODE_EXE) $(NODE_G_EXE) out/$(BUILDTYPE)/$(NODE_EXE) blog.html email.md
-	-find out/ -name '*.o' -o -name '*.a' | xargs rm -rf
+	@if [ -d out ]; then find out/ -name '*.o' -o -name '*.a' | xargs rm -rf; fi
 	-rm -rf node_modules
 
 distclean: