Switch to `cxx_std_11` as per best current practice.
Switch to `cxx_std_11` as per best current practice.

It requires a version of CMake newer than what Ubuntu Xenial has,
but I should really have bumped up to Ubuntu Bionic a year ago...

Fixes #370.

Change-Id: I1af221e03d80da2df547c63c7ed60016a892885e
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60092
Reviewed-by: Perry Lorier <d2756efe887c28699f4587ef278926ee3b3bfae5@google.com>
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2