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>
1 file changed