cmake: Various fixes.

* Allow overriding install path with CMAKE_INSTALL_PREFIX.
* make sure js2min.py can be found when building out of source.
* Replace empty macros with void(0).
  Expressions like "debug(x) && foo()" fail if debug(x) is an empty macro.
* Make sure node-natives.h creation is deterministic.
* Fix version string.
  The copyright header made the node_version.h file larger
  than the previously set file read limit.