Support C17, C23 standards with goto-cc

We previously added the necessary language features in the C front end,
but failed to permit selection of these standards in goto-cc.

Also, GCC >= 11 and Clang >= 17 support C23-style attributes
irrespective of the configured language standard. So let's accept these
in our C front end whenever operating in GCC/Clang mode.

Fixes: #8617

Author: tautschnig

regression/ansi-c/c23_attributes1/main.c

@@ -0,0 +1,8 @@
+enum [[nodiscard]] error_t
+{
+  A,
+};
+
+int main()
+{
+}

regression/ansi-c/c23_attributes1/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+-std=c23
+^EXIT=0$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/ansi-c/scanner.l

@@ -899,8 +899,12 @@ enable_or_disable ("enable"|"disable")
 "__if_not_exists"   { return MSC_cpp_keyword(TOK_MSC_IF_NOT_EXISTS); }
 "__underlying_type" { return conditional_keyword(PARSER.cpp98, TOK_UNDERLYING_TYPE); }
 
-"[["                { if(PARSER.c23)
+"[["                { if(PARSER.c23 ||
+                         PARSER.mode==configt::ansi_ct::flavourt::GCC ||
+                         PARSER.mode==configt::ansi_ct::flavourt::CLANG)
+                      {
                         BEGIN(STD_ANNOTATION);
+                      }
                       else
                       {
                         yyless(1); // puts one [ back into stream

src/goto-cc/gcc_mode.cpp

@@ -648,6 +648,20 @@ int gcc_modet::doit()
        std_string=="gnu1x" || std_string=="c1x")
       config.ansi_c.set_c11();
 
+    if(
+      std_string == "gnu17" || std_string == "c17" || std_string == "gnu18" ||
+      std_string == "c18")
+    {
+      config.ansi_c.set_c17();
+    }
+
+    if(
+      std_string == "gnu2x" || std_string == "c2x" || std_string == "gnu23" ||
+      std_string == "c23")
+    {
+      config.ansi_c.set_c23();
+    }
+
     if(std_string=="c++11" || std_string=="c++1x" ||
        std_string=="gnu++11" || std_string=="gnu++1x" ||
        std_string=="c++1y" ||