"serial" is a placeholder Brahma platform that does not support
threads.  Active messages work and there are multiple virtual clusters,
but thread creation is illegal and synchronization has no effect.  (It
is still provided to make it easy to write code that works in either
the serial or concurrent cases.)  If the number of clusters passed in
0, then BR_init looks for an env variable "CLUSTERS", if that is not
defined it is set to 1.

Define "BR_SERIAL" to use this Brahma platform.
