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/options.nim

@@ -149,6 +149,7 @@ var
   gExperimentalMode*: bool
   newDestructors*: bool
   gDynlibOverrideAll*: bool
+  useNimNamespace*: bool
 
 type
   SymbolFilesOption* = enum