Skip the Rust newgem cargo test where magnus cannot build

Ruby master added `handle_weak_references` to `rb_data_type_struct`, and no released `magnus` compiles against it, so the generated crate fails to build on `ruby-head`. Key the skip on that field being present in `rtypeddata.h` rather than on a hardcoded Ruby version.

Author: hsbt

spec/commands/newgem_options_spec.rb

@@ -1106,6 +1106,7 @@ def installed_go?
 
       def setup_rust_env
         skip "rust toolchain of mingw is broken" if RUBY_PLATFORM.match?("mingw")
+        skip "magnus has no release supporting the extended rb_data_type_struct yet (https://github.com/matsadler/magnus/issues/173)" if rb_data_type_struct_extended?
 
         env = {
           "CARGO_HOME" => ENV.fetch("CARGO_HOME", File.join(ENV["HOME"], ".cargo")),
@@ -1119,6 +1120,12 @@ def setup_rust_env
         RbConfig::CONFIG.each {|k, v| env["RBCONFIG_#{k}"] = v }
         env
       end
+
+      def rb_data_type_struct_extended?
+        rtypeddata_h = File.join(RbConfig::CONFIG["rubyhdrdir"], "ruby", "internal", "core", "rtypeddata.h")
+
+        File.exist?(rtypeddata_h) && File.read(rtypeddata_h).include?("handle_weak_references")
+      end
     end
 
     context "--ext parameter set with go" do