grpc: refactor cxxStandard selection

GRPC is less aggressive in overriding `CMAKE_CXX_STANDARD` nowadays.
This allows us to take a less invasive approach to ensure that the
provided abseil package is configured with the same implementation
for the provided c++17 compatibility shims.
Instead of always setting `CMAKE_CXX_STANDARD`, we only do it to
override hard-coded downgrade on darwin. With that, we can use the
default configuration for abseil-cpp, reducing the number of instances
of that library in the build closure to one.

pkgs/development/libraries/google-cloud-cpp/default.nix

@@ -120,7 +120,6 @@ stdenv.mkDerivation rec {
     # this adds a good chunk of time to the build
     "-DBUILD_TESTING:BOOL=ON"
     "-DGOOGLE_CLOUD_CPP_ENABLE_EXAMPLES:BOOL=OFF"
-    "-DCMAKE_CXX_STANDARD=${grpc.cxxStandard}"
   ] ++ lib.optionals (apis != [ "*" ]) [
     "-DGOOGLE_CLOUD_CPP_ENABLE=${lib.concatStringsSep ";" apis}"
   ];