test: allow UV_EACCES in spawn_fails

Allow UV_EACCES as a successful return value when uv_spawn fails.

When the PATH environment variable contains a directory which the
user cannot access, execvp may return EACCES instead of ENOENT.

fix #1045.
build: ignore generated makefiles on android

gyp currently doesn't support the --generator-output option for android,
this is the best way to ignore generated files for projects using libuv
as a dependency.
unix, windows: add uv_loop_alive() function

Useful to know when the the event loop is empty, this can't be done with
uv_run() without possibly blocking, or running some events (which might
empty the event loop as a side-effect).