Merge #65: Fix clang "unknown warning group" errors

680776f Fix clang "unknown warning group" errors (Russell Yanofsky)

Pull request description:

  Use clang __has_warning macro to avoid `unknown warning group '-Wsuggest-override'` warning in older versions of clang that have the `__has_warning` macro but don't have the `suggest-override` warning.

  Errors were showing up https://github.com/bitcoin/bitcoin/pull/10102/checks?check_run_id=4671391974 / https://cirrus-ci.com/task/4642955743985664

Top commit has no ACKs.

Tree-SHA512: 8f15376f7ec054640c417174f260f21b7406a73d0c9ebf60a17a3a5bb497cf7baeab1335ec66d8e41117460b6b5ab872ec8c00a9afb63c98602031b95230194f

Author: ryanofsky

src/mp/gen.cpp

@@ -213,7 +213,11 @@ void Generate(kj::StringPtr src_prefix,
     h << "#include <" << PROXY_DECL << ">\n\n";
     h << "#if defined(__GNUC__)\n";
     h << "#pragma GCC diagnostic push\n";
+    h << "#if !defined(__has_warning)\n";
     h << "#pragma GCC diagnostic ignored \"-Wsuggest-override\"\n";
+    h << "#elif __has_warning(\"-Wsuggest-override\")\n";
+    h << "#pragma GCC diagnostic ignored \"-Wsuggest-override\"\n";
+    h << "#endif\n";
     h << "#endif\n";
     h << "namespace mp {\n";