Work around a bug in CMake that breaks `^` anchors.

https://gitlab.kitware.com/cmake/cmake/-/issues/16899 was reported
seven years ago. The bug existed at least ten years ago. It hasn't
been fixed.

Change-Id: Ifbe3e77ce6371091b24303d5c4953d401ed29826
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63071
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>