Revert to using -stdlib=libc++ as part of CXX under OS X

Putting it in LDFLAGS breaks compilation of plugins using shared-ld-sh script
for linking as it doesn't support this option.

configure

@@ -20419,9 +20419,7 @@ if test "x$wxUSE_MACOSX_VERSION_MIN" != "x"; then
 fi
 
 if test "$HAVE_CXX11" = "1" ; then
-                CXXFLAGS="$CXXFLAGS -stdlib=libc++"
-    OBJCXXFLAGS="$OBJCXXFLAGS -stdlib=libc++"
-    LDFLAGS="$LDFLAGS -stdlib=libc++"
+                        eval "CXX=\"$CXX -stdlib=libc++\""
 fi
 
 if test "x$retest_macosx_linking" = "xyes"; then