Fix Abseil build command (#1350)
Fix Abseil build command (#1350)

@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)
damonkohler/cartographer,wohe/cartographer,damonkohler/cartographer,larics/cartographer,damonkohler/cartographer,damonkohler/cartographer,wohe/cartographer,cartographer-project/cartographer,cartographer-project/cartographer,googlecartographer/cartographer,larics/cartographer,cartographer-project/cartographer,googlecartographer/cartographer,googlecartographer/cartographer,wohe/cartographer,cartographer-project/cartographer,larics/cartographer,larics/cartographer,wohe/cartographer,googlecartographer/cartographer