goto-gcc: select default language standard depending on emulated compiler

This ensures that goto-cc with no `-std` explicitly specified will expect the same
language standard as the underlying compiler's preprocessor -- so for example, using
`goto-cc test.cpp`, which invokes `g++-7 -E` to perform preprocessing, will be able to
parse the C++14 constructs in the standard library exposed because g++-6 and above
enable C++14 support by default.

Author: smowton

src/goto-cc/gcc_mode.cpp

@@ -597,6 +597,69 @@ int gcc_modet::doit()
     if(std_string=="gnu++14" || std_string=="c++14")
       config.cpp.set_cpp14();
   }
+  else
+  {
+    // Set default dialect dependent on what compiler we're emulating:
+    switch(gcc_version.flavor)
+    {
+    case gcc_versiont::flavort::UNKNOWN:
+      // Leave configt's default in place
+      break;
+
+    case gcc_versiont::flavort::CLANG:
+      if(gcc_version.v_major >= 6)
+      {
+        // NOLINTNEXTLINE(whitespace/line_length)
+        // https://releases.llvm.org/6.0.0/tools/clang/docs/ReleaseNotes.html#c-language-changes-in-clang
+        config.cpp.set_cpp14();
+      }
+      else
+      {
+        config.cpp.set_cpp98();
+      }
+
+      if(gcc_version.v_major >= 3 && gcc_version.v_minor >= 6)
+      {
+        // NOLINTNEXTLINE(whitespace/line_length)
+        // http://releases.llvm.org/3.6.0/tools/clang/docs/ReleaseNotes.html#c-language-changes-in-clang
+        config.ansi_c.set_c11();
+      }
+      else
+      {
+        config.ansi_c.set_c99();
+      }
+
+      break;
+
+    case gcc_versiont::flavort::GCC:
+      if(gcc_version.v_major >= 6)
+      {
+        // https://gcc.gnu.org/gcc-6/changes.html
+        config.cpp.set_cpp14();
+      }
+      else
+      {
+        config.cpp.set_cpp98();
+      }
+
+      if(gcc_version.v_major >= 5)
+      {
+        // https://gcc.gnu.org/gcc-5/changes.html
+        config.ansi_c.set_c11();
+      }
+      else
+      {
+        config.ansi_c.set_c89();
+      }
+
+      break;
+
+    case gcc_versiont::flavort::BCC:
+      config.ansi_c.set_c89();
+
+      break;
+    }
+  }
 
   // gcc's default is 32 bits for wchar_t
   if(cmdline.isset("short-wchar"))