Set `CMAKE_JS_SRC`

[kraenhansen]

To be fully cmake-js compatible, we should inject CMAKE_JS_SRC: https://github.com/cmake-js/cmake-js/blob/f257177211ab685ef98092f6b4ba3c01e8e9da64/lib/cMake.js#L136-L137

On when building on Windows CMAKE_JS_SRC points to win_delay_load_hook.cc to allow the node.exe to be renamed (to support electron).