Move setting required -stdlib argument into configure.ac

Examples/Makefile.in

@@ -25,7 +25,7 @@ TARGET     =
 CC         = @CC@
 CXX        = @CXX@
 CFLAGS     = @PLATCFLAGS@
-CXXFLAGS   = @BOOST_CPPFLAGS@ @PLATCXXFLAGS@ $(filter -stdlib%,@CXXFLAGS@)
+CXXFLAGS   = @BOOST_CPPFLAGS@ @PLATCXXFLAGS@
 prefix     = @prefix@
 exec_prefix= @exec_prefix@
 SRCS       =

configure.ac

@@ -352,6 +352,15 @@ if test x"$enable_cpp11_testing" = xyes; then
   fi
 fi
 
+# On darwin before Mavericks when using clang, need to ensure using
+# libc++ for tests and examples to run under mono
+case $host in
+  *-*-darwin*) if test "$CXX" = "clang++";
+    then PLATCXXFLAGS="$PLATCXXFLAGS -stdlib=libc++"
+    fi;;
+  *) ;;
+esac
+
 # Set info about shared libraries.
 AC_SUBST(SO)
 AC_SUBST(LDSHARED)