Pthreads: POSIX threads

To enable multithreading support within Cactus using POSIX threads, the configure option is

PTHREADS = yes

The configuration process will check if a reentrant C library is available, and adds it to the linker flags. It will also search for the system's Pthreads library (either libpthread or libpthreads), and set preprocessor defines necessary for compiling multithreaded code.