Fix 'make doc' issue on systems with node & nodejs

On systems with node installed as both 'node' and 'nodejs', 'make'
will evaluate 'which node nodejs' to include both the path to 'node'
and the path to 'nodejs', ultimatley psasing 'nodejs' as the first
argument to 'node' and throwing an error.

In order to fix this, and make 'make doc' run on systems with node
aliased to both 'node' and 'nodejs', I just added a 'grep' to select
the first of 'node' and 'nodejs' if both are defined.

Tested on Ubuntu 13.10 with GNU Make 3.81, GNU bash 4.2.45(1)-release
and Node.js v0.10.26.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>