build: configure return exit status from gyp

Previously, 'configure' would not return an exit status
if gyp blows up. This can be tested via:

    date >> node.gyp ; ./configure && echo A-OK

You will get "A-OK" even though gyp had failed.

Signed-off-by: Julien Gilli <julien.gilli@joyent.com>

configure

@@ -730,4 +730,4 @@ else:
 
 gyp_args += args
 
-subprocess.call(gyp_args)
+sys.exit(subprocess.call(gyp_args))