contrib: Make flock optional.

* contrib/src/main.mak (TARBALLS_DEFAULT): New variable.
(TARBALLS): Define in terms of the above.
(FLOCK, FLOCK_PREFIX): New variables.
(download): Use $(FLOCK_PREFIX), which may be empty when flock is not
available and a default TARBALLS location is used.

Change-Id: I95ca0ec0cdd893fc50f3924619cc87fe8cd6a5b7