Print warning on OSX when using old libstdc++ (rather than libc++).

c++/src/kj/common.h

@@ -30,6 +30,7 @@
 #ifndef KJ_COMMON_H_
 #define KJ_COMMON_H_
 
+#ifndef KJ_NO_COMPILER_CHECK
 #if __cplusplus < 201103L && !__CDT_PARSER__
   #error "This code requires C++11. Either your compiler does not support it or it is not enabled."
   #ifdef __GNUC__
@@ -47,6 +48,10 @@
                "version 4.2 (wat?), is actually built from some random SVN revision between 3.1 "\
                "and 3.2.  Unfortunately, it is insufficient for compiling this library.  You can "\
                "download the real Clang 3.2 (or newer) from the Clang web site."
+    #elif __cplusplus >= 201103L && !__has_include(<initializer_list>)
+      #warning "Your compiler supports C++11 but your C++ standard library does not.  If your "\
+               "system has libc++ installed (as should be the case on e.g. Mac OSX), try adding "\
+               "-stdlib=libc++ to your CXXFLAGS."
     #endif
   #else
     #if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
@@ -55,6 +60,11 @@
   #endif
 #elif defined(_MSC_VER)
   #warning "As of June 2013, Visual Studio's C++11 support was hopelessly behind what is needed to compile this code."
+#else
+  #warning "I don't recognize your compiler.  As of this writing, Clang and GCC are the only "\
+           "known compilers with enough C++11 support for this library.  "\
+           "#define KJ_NO_COMPILER_CHECK to make this warning go away."
+#endif
 #endif
 
 // =======================================================================================