Don't try to make binaries that run on ancient OS when using current tools

For some reason the current Clang linker doesn't like that and
misbehaves, Apple bug 12075544. It is unclear whether binaries built
using a modern Xcode and SDK to run on 10.4 actually would run on 10.4
anyway.

Change-Id: Id15da5eaa4bb95af9cf6d98a658c510f4a786185

configure.in

@@ -2693,7 +2693,12 @@ if test "$_os" = "Darwin"; then
     AC_MSG_RESULT([SDK $with_macosx_sdk at $MACOSX_SDK_PATH])
 
     if test "$with_macosx_version_min_required" = ""; then
-        with_macosx_version_min_required="10.4"
+        case $with_macosx_sdk in
+        10.4|10.5)
+            with_macosx_version_min_required="10.4";;
+        *)
+            with_macosx_version_min_required="10.6";;
+        esac
     fi
 
     if test "$with_macosx_version_max_allowed" = ""; then