openmp-simd flags for icc and gcc

To use #pragma omp simd to vectorise loops in both gcc and icc, the code need to be compiled with additional compiler flags: -fopenmp-simd (gcc) and -qopenmp-simd (icc).

However, -fopenmp-simd is only supported from gcc version 4.9.1 and above.

The flags should be enabled by default when vectorisation is on and disabled when using --disable-vec.