conditional setting of CXXFLAGS in Makefile

[iamthebot]

-O3 as default, keep -std separate in case of an older compiler (they can use -e to override in that case).

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[iamthebot]

I signed it (CLA signed under corporation Blacklab Capital)!

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.

[iamthebot]

This should be under the same CLA (corporation: Blacklab Capital)

[iamthebot]

@googlebot I've updated the agreement. Should include my github username.

[iamthebot]

Closing in order to start off a clean branch.