Use dedicated source file for rootcling stage 1 and 2.

Now that these files are mostly preprocessor directives, splitting makes sense.

main/src/rootcling.cxx

+// Authors: Axel Naumann, Philippe Canal, Danilo Piparo
+
+/*************************************************************************
+ * Copyright (C) 1995-2016, Rene Brun and Fons Rademakers.               *
+ * All rights reserved.                                                  *
+ *                                                                       *
+ * For the licensing terms see $ROOTSYS/LICENSE.                         *
+ * For the list of contributors see $ROOTSYS/README/CREDITS.             *
+ *************************************************************************/
+
+#ifndef ROOT_STAGE1_BUILD
+#include "rootclingTCling.h"
+#include "rootclingIO.h"
+#endif
+
+#include "rootcling_impl.h"
+#include "RConfigure.h"
+#include "RConfig.h"
+
+#include <iostream>
+
+#ifdef _WIN32
+# ifdef system
+#  undef system
+# endif
+# include <windows.h>
+# include <Tlhelp32.h> // for MAX_MODULE_NAME32
+# include <process.h>
+# define PATH_MAX _MAX_PATH
+# ifdef interface
+// prevent error coming from clang/AST/Attrs.inc
+#  undef interface
+# endif
+#else // _WIN32
+# include <limits.h>
+# include <unistd.h>
+# include <dlfcn.h>
+#endif
+
+#ifdef __APPLE__
+#include <libgen.h> // Needed for basename
+#include <mach-o/dyld.h>
+#endif
+
+extern "C" {
+R__DLLEXPORT void usedToIdentifyRootClingByDlSym() {}
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+#ifdef __ICC
+#pragma warning disable 69
+#endif
+
+
+ROOT::Internal::RootCling::TROOTSYSSetter gROOTSYSSetter;
+
+int main(int argc, char **argv)
+{
+   // Force the emission of the symbol - the compiler cannot know that argv
+   // is always set.
+   if (!argv) {
+      auto dummyVal =  (int)(long)&usedToIdentifyRootClingByDlSym;
+      return dummyVal;
+   }
+
+   ROOT::Internal::RootCling::DriverConfig config{};
+#ifdef R__HAVE_LLVMRESOURCEDIR
+  config.fLLVMResourceDir= "@R__LLVMRESOURCEDIR@";
+#endif
+
+#ifdef ROOT_STAGE1_BUILD
+   config.fBuildingROOTStage1 = true;
+#else
+   config.fBuildingROOTStage1 = false;
+   config.fTROOT__GetExtraInterpreterArgs = &TROOT__GetExtraInterpreterArgs;
+   config.fTCling__GetInterpreter = &TCling__GetInterpreter;
+   config.fInitializeStreamerInfoROOTFile = &InitializeStreamerInfoROOTFile;
+   config.fAddStreamerInfoToROOTFile = &AddStreamerInfoToROOTFile;
+   config.fAddTypedefToROOTFile = &AddTypedefToROOTFile;
+   config.fAddEnumToROOTFile = &AddEnumToROOTFile;
+   config.fAddAncestorPCMROOTFile = &AddAncestorPCMROOTFile;
+   config.fCloseStreamerInfoROOTFile = &CloseStreamerInfoROOTFile;
+#endif
+   return ROOT_rootcling_Driver(argc, argv, config);
+}