Makes `common::Mutex` an alias for `absl::Mutex` and replaces the logic inside `common::MutexLocker` with `absl::MutexLock`.
A future PR can then remove these classes entirely and replace references to it across the Cartographer code base with the Abseil classes directly.
@pifon2a I've noticed the `BUILD_COMMAND` is broken. You're setting `ABSL_BUILD_COMMAND`, but using `ABSEIL_BUILD_COMMAND` (effectively empty). Somehow this doesn't confuse newer CMakes, but the one you install on Trusty gets confused and interprets this as a no-op build step. This causes the indigo pipeline in cartographer_ros to fail.
Use the cross-platform build command instead:
https://cmake.org/cmake/help/latest/module/ExternalProject.html#obtaining-project-properties (just above the linked paragraph)
Alexander Belyaev
Arno Fleck
Juraj Oršulić