abseil exposes different interfaces depending on what is available in
`std` in a given C++ standard. gcc 16 defaults to C++20, thus causing
the default abseil-cpp to expose a different interface than the one
webrtc-audio-processing (built with C++17) expects. this leads to a
great deal of "error: 'partial_ordering' has not been declared in 'std'"
and similar originating from abseil's types/compare.h. thus, we
explicitly override abseil to specify the desired C++ standard.
