Automatically add -stdlib=libc++ if system C++ library doesn't support C++11 (needed on OSX).

c++/m4/ax_cxx_compile_stdcxx_11.m4

@@ -16,10 +16,16 @@
 #   If neither is specified, you get whatever works, with preference for an
 #   extended mode.
 #
+#   Additionally, check if the standard library supports C++11.  If not,
+#   try adding -stdlib=libc++ to see if that fixes it.  This is needed e.g.
+#   on Mac OSX 10.8, which ships with a very old libstdc++ but a relatively
+#   new libc++.
+#
 # LICENSE
 #
 #   Copyright (c) 2008 Benjamin Kosnik <bkoz@redhat.com>
 #   Copyright (c) 2012 Zack Weinberg <zackw@panix.com>
+#   Copyright (c) 2013 Kenton Varda <temporal@gmail.com>
 #
 #   Copying and distribution of this file, with or without modification, are
 #   permitted in any medium without royalty provided the copyright notice
@@ -45,6 +51,13 @@ m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody], [
     check_type&& cr = static_cast<check_type&&>(c);
 ])
 
+m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody_lib], [
+  #include <initializer_list>
+  #include <unordered_map>
+  #include <atomic>
+  #include <thread>
+])
+
 AC_DEFUN([AX_CXX_COMPILE_STDCXX_11], [dnl
   m4_if([$1], [], [],
         [$1], [ext], [],
@@ -103,5 +116,35 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX_11], [dnl
 
   if test x$ac_success = xno; then
     AC_MSG_ERROR([*** A compiler with support for C++11 language features is required.])
+  else
+    ac_success=no
+    AC_CACHE_CHECK(whether $CXX supports C++11 library features by default,
+                   ax_cv_cxx_compile_cxx11_lib,
+      [AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody_lib])],
+         [ax_cv_cxx_compile_cxx11_lib=yes],
+         [ax_cv_cxx_compile_cxx11_lib=no])
+      ])
+    if test x$ax_cv_cxx_compile_cxx11_lib = xyes; then
+      ac_success=yes
+    else
+      # Try with -stdlib=libc++
+      AC_CACHE_CHECK(whether $CXX supports C++11 library features with -stdlib=libc++,
+                     ax_cv_cxx_compile_cxx11_lib_libcxx,
+        [ac_save_CXXFLAGS="$CXXFLAGS"
+         CXXFLAGS="$CXXFLAGS -stdlib=libc++"
+         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody_lib])],
+          [eval ax_cv_cxx_compile_cxx11_lib_libcxx=yes],
+          [eval ax_cv_cxx_compile_cxx11_lib_libcxx=no])
+         CXXFLAGS="$ac_save_CXXFLAGS"])
+      if eval test x$ax_cv_cxx_compile_cxx11_lib_libcxx = xyes; then
+        CXXFLAGS="$CXXFLAGS -stdlib=libc++"
+        ac_success=yes
+        break
+      fi
+    fi
+
+    if test x$ac_success = xno; then
+      AC_MSG_ERROR([*** A C++ library support for C++11 features is required.])
+    fi
   fi
 ])