G M: Provides the _LIBCPP_WARNING macro, to be used for MSVC only, since that compiler doesn't support #warning.

llvm-svn: 191980
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 80b84d4c268b75994524b00db7a1437b070e98a8
diff --git a/src/thread.cpp b/src/thread.cpp
index cdfa2fd..338a8a2 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -89,7 +89,11 @@
 #else  // defined(CTL_HW) && defined(HW_NCPU)
     // TODO: grovel through /proc or check cpuid on x86 and similar
     // instructions on other architectures.
-#warning hardware_concurrency not yet implemented
+#   if defined(_MSC_VER) && ! defined(__clang__)
+        _LIBCPP_WARNING("hardware_concurrency not yet implemented")
+#   else
+#       warning hardware_concurrency not yet implemented
+#   endif
     return 0;  // Means not computable [thread.thread.static]
 #endif  // defined(CTL_HW) && defined(HW_NCPU)
 }