Include -lpthread on the link line automatically only for C++

lib/Driver/Tools.cpp

 //===--- Tools.cpp - Tools Implementations --------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "Tools.h"
@@ skipping 7699 matching lines @@
       CmdArgs.push_back("-Bdynamic");
     CmdArgs.push_back("-lm");
   }
 
   if (!Args.hasArg(options::OPT_nostdlib)) {
     if (!Args.hasArg(options::OPT_nodefaultlibs)) {
       // Always use groups, since it has no effect on dynamic libraries.
       CmdArgs.push_back("--start-group");
       CmdArgs.push_back("-lc");
       // libc++ and PPAPI programs always require libpthread, so just always
-      // include it in the group.
-      Args.ClaimAllArgs(options::OPT_pthread);
-      Args.ClaimAllArgs(options::OPT_pthreads);
-      CmdArgs.push_back("-lpthread");
+      // include it in the group for C++.
+      if (Args.hasArg(options::OPT_pthread) ||
+          Args.hasArg(options::OPT_pthreads) ||
+          D.CCCIsCXX()) {
+        CmdArgs.push_back("-lpthread");
+      }
 
       CmdArgs.push_back("-lgcc");
       CmdArgs.push_back("--as-needed");
       if (IsStatic)
         CmdArgs.push_back("-lgcc_eh");
       else
         CmdArgs.push_back("-lgcc_s");
       CmdArgs.push_back("--no-as-needed");
       CmdArgs.push_back("--end-group");
     }
@@ skipping 761 matching lines @@
       CmdArgs.push_back("-lmsvcrt");
       AddRunTimeLibs(TC, D, CmdArgs, Args);
     }
   }
 
   const std::string Linker = TC.GetProgramPath("ld");
   Exec = Args.MakeArgString(Linker);
 
   C.addCommand(llvm::make_unique<Command>(JA, *this, Exec, CmdArgs));
 }
-