Jose's updates

[varunagrawal]

Adding the updates @jlblancoc made as per #1174 (comment)

[ProfFan]

I think march=native on is a good default, errors related to this are probably deeper issues like different compilation flags that is misunderstood to be a march issue.

I think the Eigen change is good :)

[varunagrawal]

I think march=native on is a good default, errors related to this are probably deeper issues like different compilation flags that is misunderstood to be a march issue.

I am no expert on architecture specific code generation, but @jlblancoc seemed to think setting march=native off was a good idea.

From a more practical standpoint, I have noticed issues when compiling downstream repos (e.g. GTDynamics, GPMP2 etc) when the march=native flag is on, and these are incredibly hard to debug. For this reason, I still think the default should be OFF and those who want the extra boost from architecture specific machine code can turn it on (aka generality over performance).