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/cgen.nim

@@ -676,6 +676,12 @@ proc generateHeaders(m: BModule) =
   add(m.s[cfsHeaders], "#undef powerpc" & tnl)
   add(m.s[cfsHeaders], "#undef unix" & tnl)
 
+proc openNamespaceNim(): Rope =
+  result.add("namespace Nim {" & tnl)
+  
+proc closeNamespaceNim(): Rope =
+  result.add("}" & tnl)
+
 proc closureSetup(p: BProc, prc: PSym) =
   if tfCapturesEnv notin prc.typ.flags: return
   # prc.ast[paramsPos].last contains the type we're after:
@@ -918,6 +924,7 @@ proc addIntTypes(result: var Rope) {.inline.} =
   addf(result, "#define NIM_NEW_MANGLING_RULES" & tnl &
                "#define NIM_INTBITS $1" & tnl, [
     platform.CPU[targetCPU].intSize.rope])
+  if useNimNamespace : result.add("#define USE_NIM_NAMESPACE" & tnl)
 
 proc getCopyright(cfile: Cfile): Rope =
   if optCompileOnly in gGlobalOptions:
@@ -1083,7 +1090,11 @@ proc genMainProc(m: BModule) =
   appcg(m, m.s[cfsProcs], nimMain,
         [m.g.mainModInit, initStackBottomCall, rope(m.labels)])
   if optNoMain notin gGlobalOptions:
+    if useNimNamespace: 
+      m.s[cfsProcs].add closeNamespaceNim() & "using namespace Nim;" & tnl
+
     appcg(m, m.s[cfsProcs], otherMain, [])
+    if useNimNamespace: m.s[cfsProcs].add openNamespaceNim()
 
 proc getSomeInitName(m: PSym, suffix: string): Rope =
   assert m.kind == skModule
@@ -1191,7 +1202,9 @@ proc genModule(m: BModule, cfile: Cfile): Rope =
     add(result, genSectionStart(i))
     add(result, m.s[i])
     add(result, genSectionEnd(i))
+    if useNimNamespace and i == cfsHeaders: result.add openNamespaceNim()    
   add(result, m.s[cfsInitProc])
+  if useNimNamespace: result.add closeNamespaceNim()
 
 proc newPreInitProc(m: BModule): BProc =
   result = newProc(nil, m)
@@ -1330,11 +1343,13 @@ proc writeHeader(m: BModule) =
     add(result, genSectionStart(i))
     add(result, m.s[i])
     add(result, genSectionEnd(i))
+    if useNimNamespace and i == cfsHeaders: result.add openNamespaceNim()
   add(result, m.s[cfsInitProc])
 
   if optGenDynLib in gGlobalOptions:
     result.add("N_LIB_IMPORT ")
   result.addf("N_CDECL(void, NimMain)(void);$n", [])
+  if useNimNamespace: result.add closeNamespaceNim()
   result.addf("#endif /* $1 */$n", [guard])
   writeRope(result, m.filename)