Fix Abseil usage of installed cmake-config

Fix the generated cmake-config file to look for the used Abseil-cpp
config files providing the needed targets.

When using Abseil from system (or locally installed and set via
`absl_DIR`) the used targets like `absl::log` need to be made available
in downstream projects using `ceres-solver`.

For system packages with no `absl_VERSION` info check for the required
target `absl::absl_vlog_is_on`, just like we do in `CMakeLists.txt`.

Error on installation/export of "no-absl-VERSION" as we can't check
version mismatches.

When using `abseil-cpp` as submodule the abseil-configs are added to the
install target as well. Use these config files to make the needed Abseil
targets available.

In the submodule case extract the `absl_VERSION` variable from its
`CMakeLists.txt` file for us to check against in the generated
cmake-config-file.

Fixes: https://github.com/ceres-solver/ceres-solver/issues/1089

Change-Id: I3e3d079ddf931e7fe1b57783471c32c19e4d31b6