individual abseil-cpp_* versions allow overriding the C++ standard
version that abseil is built with. this is very useful when the default
compiler C++ standard is different than the project's C++ standard.
accordingly, we extend this capability to the top-level abseil-cpp
wrapper alias as there's no reason to pin to an abseil LTS just for
this.
Some packages, such as or-tools, strictly require a specific LTS branch
of abseil-cpp, and will be broken by arbitrary upgrades. See also
https://abseil.io/about/compatibility, which confirms “Each LTS release
should be considered to be a new major version of the library.”
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
