Allow Clang to compile without libc++

Clang is perfectly able to deal with the libstdc++ headers. The
problem on Mac is that those headers are mightily old (from GCC 4.2),
so they are insufficient for C++11 support.

So make a more accurate test. This allows Clang to enable C++11 in the
presence of newer libstdc++ header (e.g., Clang on Linux or Clang on
FreeBSD).

Change-Id: I4f457ca82bf13feca0af78c9363cb6365bb3f68e
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

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

@@ -46,8 +46,12 @@
 #endif
 
 #include <utility>
-#if defined(__clang__) && !defined(_LIBCPP_VERSION)
-#  error "C++11 with clang requires libc++ runtime"
+#if defined(__clang__)
+#  if __has_feature(cxx_generalized_initializers)
+//   On Mac OS X, the libstdc++ headers don't include <initializer_list>
+//   This #include here forces a failure unless we're using libc++
+#    include <initializer_list>
+#  endif
 #endif
 
 int main(int, char **) { return 0; }