Make AX_CXX_COMPILE_STDCXX_11 test for -stdlib=libc++ via std::shared_ptr

The clang shipped with OSX XCode and clangs not build enabling libcpp,
will default to the libstdc++ headers and lib installed on the system.
In the OSX case, that libstdc++ is the one bundles with gcc-4.2, which
is far too old to provide all required C++11 types, such as
std::shared_ptr.
Hence, the C++11 check should try to compile a program with a C++11 type
and try -stdlib=libc++ if the default lib fails to compile said program.

m4/ax_cxx_compile_stdcxx_11.m4

@@ -22,11 +22,16 @@
 #   'optional', then configuration proceeds regardless, after defining
 #   HAVE_CXX11 if and only if a supporting mode is found.
 #
+#   This version of AX_CXX_COMPILE_STDCXX_11 will also check if specifying
+#   `-stdlib=libc++`` is required, as it is on current OSX systems using
+#   a clang which defaults to an old libstdc++.
+#
 # LICENSE
 #
 #   Copyright (c) 2008 Benjamin Kosnik <bkoz@redhat.com>
 #   Copyright (c) 2012 Zack Weinberg <zackw@panix.com>
 #   Copyright (c) 2013 Roy Stogner <roystgnr@ices.utexas.edu>
+#   Copyright (c) 2013 Nils Maier <maierman@web.de>
 #
 #   Copying and distribution of this file, with or without modification, are
 #   permitted in any medium without royalty provided the copyright notice
@@ -34,8 +39,11 @@
 #   warranty.
 
 #serial 3
+#modified to check fo -stdlib=libc++ (required on OSX)
 
 m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody], [
+  #include <memory>
+
   template <typename T>
     struct check
     {
@@ -52,6 +60,9 @@ m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody], [
     check_type&& cr = static_cast<check_type&&>(c);
 
     auto d = a;
+
+    // Check std::shared_ptr is available, which might require -stdlib=libc++.
+    std::shared_ptr<check_type> ptr;
 ])
 
 AC_DEFUN([AX_CXX_COMPILE_STDCXX_11], [dnl
@@ -77,41 +88,45 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX_11], [dnl
   m4_if([$1], [noext], [], [dnl
   if test x$ac_success = xno; then
     for switch in -std=gnu++11 -std=gnu++0x; do
-      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx11_$switch])
+      for lib in "" -stdlib=libc++; do
-      AC_CACHE_CHECK(whether $CXX supports C++11 features with $switch,
+        cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx11_$switch$lib])
+        AC_CACHE_CHECK(whether $CXX supports C++11 features with $switch $lib,
                       $cachevar,
           [ac_save_CXXFLAGS="$CXXFLAGS"
-         CXXFLAGS="$CXXFLAGS $switch"
+          CXXFLAGS="$CXXFLAGS $switch $lib"
           AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody])],
             [eval $cachevar=yes],
             [eval $cachevar=no])
           CXXFLAGS="$ac_save_CXXFLAGS"])
         if eval test x\$$cachevar = xyes; then
-        CXXFLAGS="$CXXFLAGS $switch"
+          CXXFLAGS="$CXXFLAGS $switch $lib"
           ac_success=yes
-        break
+          break 2
         fi
       done
+    done
   fi])
 
   m4_if([$1], [ext], [], [dnl
   if test x$ac_success = xno; then
     for switch in -std=c++11 -std=c++0x; do
-      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx11_$switch])
+      for lib in "" -stdlib=libc++; do
-      AC_CACHE_CHECK(whether $CXX supports C++11 features with $switch,
+        cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx11_$switch$lib])
+        AC_CACHE_CHECK(whether $CXX supports C++11 features with $switch $lib,
                       $cachevar,
           [ac_save_CXXFLAGS="$CXXFLAGS"
-         CXXFLAGS="$CXXFLAGS $switch"
+          CXXFLAGS="$CXXFLAGS $switch $lib"
           AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_11_testbody])],
             [eval $cachevar=yes],
             [eval $cachevar=no])
           CXXFLAGS="$ac_save_CXXFLAGS"])
         if eval test x\$$cachevar = xyes; then
-        CXXFLAGS="$CXXFLAGS $switch"
+          CXXFLAGS="$CXXFLAGS $switch $lib"
           ac_success=yes
-        break
+          break 2
         fi
       done
+    done
   fi])
   AC_LANG_POP([C++])
   if test x$ax_cxx_compile_cxx11_required = xtrue; then