Get node version from node executable instead of keeping it hardcoded to
the Makefile.

Current version of npm install installs a link to the build directory.
The workaround is to use npm pack to make a tarball and install from
that.
npm also adds useless metadata to package.json exposing
PKG_BUILD_DIR, so it needs to be stripped.

Signed-off-by: Eneas U de Queiroz <cote2004-github@yahoo.com>