build: Use Gradle daemon mode for 'make clean'

Before:

  [penberg@localhost osv]$ time make clean
    CLEAN
    GRADLE CLEAN

  real	0m4.026s
  user	0m8.732s
  sys	0m0.212s

After:

  [penberg@localhost osv]$ time make clean
    CLEAN
    GRADLE CLEAN

  real	0m1.658s
  user	0m0.962s
  sys	0m0.162s

Signeg-off-by: Pekka Enberg <penberg@cloudius-systems.com>