[nacltoons] Don't built with nacl-mounts by default

The bots don't have nacl-mounts installed, nor to they
build it as part of the game build anymore.

TEST=make all
BUG=None

Review URL: https://codereview.chromium.org/12316137

git-svn-id: https://nativeclient-sdk.googlecode.com/svn/trunk/src@1512 050acbb0-2703-11df-ab0a-9f3f633ae91d