Nim namespace for cpp (#7453)

- new option --usenamespace to generate nim cpp code in its own namespace Nim
- minor changes for compatibility with the new Embarcadero = Borland C++-Builder (bcc)

compiler/commands.nim

@@ -698,6 +698,11 @@ proc processSwitch(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     expectNoArg(switch, arg, pass, info)
     newDestructors = true
     defineSymbol("nimNewRuntime")
+  of "cppcompiletonamespace":
+    expectNoArg(switch, arg, pass, info)
+    useNimNamespace = true
+    defineSymbol("cppCompileToNamespace")
+    
   else:
     if strutils.find(switch, '.') >= 0: options.setConfigVar(switch, arg)
     else: invalidCmdLineOption(pass, switch, info)