support both "node" and "nodejs" as nodejs executables

Nowadays, Debian comes with 'nodejs' instead of 'node' as nodejs executable. When compiling an opa project, the .exe file will incorrectly assume that nodejs is some executable called 'node'.

The changes in the configure script enable compiling opa itself on systems where nodejs is called 'nodejs'.
The changes in launch_helper.sh enable projects to run out of the box from the .exe opa creates on systems where nodejs is called 'nodejs'.