Enable/Disable Vectorisation

I think it would be useful if we were able to turn auto-vectorisation on and off at the configuration stage. For example use:

./configure --enable-vec

to turn vectorisation on (auto-vectorisation occurs by default at -O2 and higher optimisations)

and either

./configure --enable-vec=no 

or

./configure --disable-vec 

to turn vectorisation off.

The compiler flags required to disable vectorisation in icc are:

-no-vec -no-simd 

I am still looking into the gcc compiler flags but I think:

-fno-tree-vectorize

disables auto-vectorisation.

Is this doable @pdraper?