Mix of changes from aquila

Fit into 80 columns. Only use PTHREAD_THREADS_MAX if it is defined.
Argument to control the matrix seed
Fix the threading!

Needed barriers or our results were different to serial.
Also comments!
Dang code blocks
Last changes from aquila

Command line option for testing correctness.
Small threading changes.
Limit random array values to 10000.
Use timeofday() instead of clock()