clang: Do not use libstdc++ when C++11 is enabled

We expect that the C++ library also supports C++11 when the compiler
says it does as well. When using clang with libstdc++, the library has
no C++11 support or headers, so we cannot use, for example, #include
<initializer_lists>.

Change-Id: I844877107256f15ab314e8f82fe54434dbb07638
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

config.tests/unix/c++11/c++11.cpp

@@ -42,7 +42,12 @@
 #if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)
 // Compiler claims to support C++11, trust it
 #else
-#  error "__cplusplus must be >= 201103L"
+#  error "__cplusplus must be >= 201103L, or __GXX_EXPERIMENTAL_CXX0X__ must be defined"
+#endif
+
+#include <utility>
+#if defined(__clang__) && !defined(_LIBCPP_VERSION)
+#  error "C++11 with clang requires libc++ runtime"
 #endif
 
 int main(int, char **) { return 0; }