Please allow using system abseil (over bundled abseil)

[Allen-Webb]

On ChromeOS we provide abseil-cpp as a system library. We have hacks to get autofdo building against the system abseil, but it would be preferable if there were officially supported CMake config options to support this.

Here is the CL were we hack around the bundled copy:
https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/4895161

The key parts of the hack are:

1. Removing the bundled abseil
2. Symlinking the system headers so the include paths in the source files are correct
3. Replacing add_subdirectory(third_party/abseil) with find_package(absl REQUIRED)

Officially supporting it would probably look a bit different.