Eliminate spurious error message from which.

Some versions of `which` (Cygwin's among them) spit out an error to stderr when not all their args are found. We only expect one of these to be found anyway.

Makefile

@@ -5,7 +5,7 @@ BUILD_DIR = build
 PREFIX = .
 DIST_DIR = ${PREFIX}/dist
 
-JS_ENGINE ?= `which node nodejs`
+JS_ENGINE ?= `which node nodejs 2>/dev/null`
 COMPILER = ${JS_ENGINE} ${BUILD_DIR}/uglify.js --unsafe
 POST_COMPILER = ${JS_ENGINE} ${BUILD_DIR}/post-compile.js