protobufc: unpin standard version to fix build with gcc 16 (#538023)

pkgs/by-name/pr/protobufc/package.nix

@@ -36,6 +36,21 @@ stdenv.mkDerivation (finalAttrs: {
     zlib
   ];
 
+  # The upstream macro is vendored from a very old autoconf archive:
+  # https://github.com/protobuf-c/protobuf-c/commit/42612b4ba4b11d48b76e3643fa6d42f617e661b6
+  # and the build system appears to arbitrarily require C++17 specifically:
+  # https://github.com/protobuf-c/protobuf-c/blob/4719fdd7760624388c2c5b9d6759eb6a47490626/configure.ac#L72
+  # However, the default standard version used by GCC continues to increase
+  # (e.g. C++20 for GCC 16), and so protobuf-c's dependencies do as well. In
+  # particular, abseil-cpp has headers that protobuf-c includes and are
+  # sensitive to the standard version. While we could override the standard
+  # version used by these dependents, it is simpler to drop the requirement and
+  # allow the compiler default standard to be used.
+  postPatch = ''
+    substituteInPlace configure.ac --replace-fail \
+      "AX_CXX_COMPILE_STDCXX(17, noext, mandatory)" ""
+  '';
+
   env.PROTOC = lib.getExe buildPackages.protobuf_33;
 
   meta = {