stack-size portfolio fix.  boost 1.50 now required

Liana and Kshitij - how does this look?

The intended behavior is this.  If Boost threading library is available and portfolio is requested, we do a try-link during configure that sees if we can set a thread attribute.  If that compiles and links, we set a #define in cvc4autoconfig.h.  There's a new option --thread-stack=N, with N given in megabytes.  0 is default for the platform (which can be based on ulimit I guess as I mentioned in an email).  If --thread-stack=N is given to sequential CVC4, it's an error.  If it's given to pcvc4 and N > 0 and there isn't support in that Boost version for it, it's an error.