Use 2014 ISO C++ standard on the host to compile abseil-cpp

This is only needed when using the cc toolchain from nixpkgs, since
it explicitly adds the flag `-std=c++0x` - see [1].

[1]: https://github.com/tweag/rules_nixpkgs/blob/4089de9a9cad10c703d0bbb1bba119ca99299e54/toolchains/cc/cc.nix#L201-L204

.bazelrc.common

@@ -41,6 +41,9 @@ build:linux-nixpkgs --config=nixpkgs
 build:macos-nixpkgs --config=nixpkgs
 build:nixpkgs --host_platform=@rules_nixpkgs_core//platforms:host
 
+# abseil c++ (a dependency of protobuf) needs a C++14 compiler, but rules_nixpkgs cc sets `-std=c++0x`
+build:nixpkgs --host_cxxopt=-std=c++14
+
 # Build and Test Filters
 # ----------------------
 # Note tag filters do not compose. If you wish to augment these filters on the