add gnu18 and gnuxx20 version variants

res/toolchain-schema.json

                 "c89",
                 "c99",
                 "c11",
-                "c18"
+                "c18",
+                "gnu18"
             ]
         },
         "cxx_version": {
                 "c++11",
                 "c++14",
                 "c++17",
-                "c++20"
+                "c++20",
+                "gnu++20"
             ]
         },
         "warning_flags": {

src/dds/toolchain/from_json.cpp

         c99,
         c11,
         c18,
+        gnu18,
     } c_version_e
         = [&] {
               if (!c_version) {
                   return c11;
               } else if (c_version == "c18") {
                   return c18;
+              } else if (c_version == "gnu18") {
+                  return gnu18;
               } else {
                   fail(context, "Unknown `c_version` ‘{}’", *c_version);
               }
         cxx14,
         cxx17,
         cxx20,
+        gnuxx20,
     } cxx_version_e
         = [&] {
               if (!cxx_version) {
                   return cxx17;
               } else if (cxx_version == "c++20") {
                   return cxx20;
+              } else if (cxx_version == "gnu++20") {
+                  return gnuxx20;
               } else {
                   fail(context, "Unknown `cxx_version` ‘{}’", *cxx_version);
               }
         {{gnu, c99}, {"-std=c99"}},
         {{gnu, c11}, {"-std=c11"}},
         {{gnu, c18}, {"-std=c18"}},
+        {{gnu, gnu18}, {"-std=gnu18"}},
         {{clang, c_none}, {}},
         {{clang, c89}, {"-std=c89"}},
         {{clang, c99}, {"-std=c99"}},
         {{clang, c11}, {"-std=c11"}},
         {{clang, c18}, {"-std=c18"}},
+        {{clang, gnu18}, {"-std=gnu18"}},
     };
 
     auto get_c_version_flags = [&]() -> string_seq {
         {{gnu, cxx14}, {"-std=c++14"}},
         {{gnu, cxx17}, {"-std=c++17"}},
         {{gnu, cxx20}, {"-std=c++20"}},
+        {{gnu, gnuxx20}, {"-std=gnu++20"}},
         {{clang, cxx_none}, {}},
         {{clang, cxx98}, {"-std=c++98"}},
         {{clang, cxx03}, {"-std=c++03"}},
         {{clang, cxx14}, {"-std=c++14"}},
         {{clang, cxx17}, {"-std=c++17"}},
         {{clang, cxx20}, {"-std=c++20"}},
+        {{clang, gnuxx20}, {"-std=gnu++20"}},
     };
 
     auto get_cxx_version_flags = [&]() -> string_seq {

src/dds/toolchain/toolchain.cpp

     CXX_VER_TAG("c++14:", "c++14");
     CXX_VER_TAG("c++17:", "c++17");
     CXX_VER_TAG("c++20:", "c++20");
+    CXX_VER_TAG("gnu++20:", "gnu++20");
 
     struct compiler_info {
         string c;