Fix for 'make' failure when process.env.JOBS is '0'

If you use the environment variable `JOBS` which are set '0' for other purpose,
`node-gpy rebuild` will fail to execute `make`.
Because the `-j` option of `make` require a positive integral argument.

```
$ npm install nightmare

...

> weak@0.3.3 install /home/kui/tmp/nm/node_modules/nightmare/node_modules/phantom/node_modules/dnode/node_modules/weak
> node-gyp rebuild

make: the `-j' option requires a positive integral argument
Usage: make [options] [target] ...
Options:
  -b, -m                      Ignored for compatibility.
  -B, --always-make           Unconditionally make all targets.
  -C DIRECTORY, --directory=DIRECTORY
                              Change to DIRECTORY before doing anything.
  -d                          Print lots of debugging information.
  --debug[=FLAGS]             Print various types of debugging information.
  -e, --environment-overrides
                              Environment variables override makefiles.
  -f FILE, --file=FILE, --makefile=FILE
                              Read FILE as a makefile.
  -h, --help                  Print this message and exit.
  -i, --ignore-errors         Ignore errors from commands.
  -I DIRECTORY, --include-dir=DIRECTORY
                              Search DIRECTORY for included makefiles.
  -j [N], --jobs[=N]          Allow N jobs at once; infinite jobs with no arg.
  -k, --keep-going            Keep going when some targets can't be made.

```