Microbenchmarks fixes (C, R, Octave)

Originally there was an issue in the R code that wasn't addressed with the last commit. The `parseintperf` function was using random numbers between 0 and 2^31-1 but it should have been 0 and 2^32-1 instead.

I added the `volatile` keyword to the fib test in `perf.c`. This should be a first step to fix #4821, but the Fortran issue is still not solved.

Using this <a href="https://github.com/JuliaLang/julia/issues/6791#issuecomment-53108229">suggestion</a>, I updated the Octave/MATLAB microbenchmark. I tested it in Octave and seems to be working. I don't have access to MATLAB, so it should be verified first that it does not break anything in MATLAB. If that's the case, this should fix #6791.