rewords configure.ac to make less ambiguous 

When gcc-g++ is not installed, the end user gets a confusing error. "TORQUE needs pthreads..."
Rewords the error message to tell the user *how* to install pthreads.

http://spuder.wordpress.com/2013/04/10/configure-error-torque-needs-pthreads-in-order-to-build/