test: make the node-weak install build agaist this repo

This prevents node-gyp from attempting to download dev header files from
nodejs.org. Instead, the files in this repo will be used to build the module.

Makefile

@@ -65,6 +65,7 @@ node_modules/weak:
 	@if [ ! -f node ]; then make all; fi
 	@if [ ! -d node_modules ]; then mkdir -p node_modules; fi
 	./node deps/npm/bin/npm-cli.js install weak \
+		--nodedir="$(shell pwd)" \
 		--prefix="$(shell pwd)" --unsafe-perm # go ahead and run as root.
 
 test-gc: all node_modules/weak