Merge pull request #43 from mdeters/threadstack

stack-size portfolio fix.  boost 1.50 now required

"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."