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 <perryl@google.com> Reviewed-by: Paul Wankadia <junyer@google.com>