Default to MAC_OS_X_VERSION_MIN_REQUIRED=1080 for Mac OS X SDK >= 10.8

...esp. after aba01069 "Always build 64-bit code
for OS X" combined with the fact that the TDF 64-bit builds state 10.8 as a
minimum requirement.

For a build with SDK >= 10.8 and no explicit --with-macosx-version-min-required,
this also enforces libc++ instead of libstdc++, which is a prerequisite for
future use of more C++11 features.

Change-Id: Idc770c30ff844b6c3e84d088b5deead0a577e7bf

configure.ac

@@ -2830,8 +2830,10 @@ if test $_os = Darwin; then
         case $with_macosx_sdk in
         10.5)
             with_macosx_version_min_required="10.5";;
-        *)
+        10.6|10.7)
             with_macosx_version_min_required="10.6";;
+        *)
+            with_macosx_version_min_required="10.8";;
         esac
     fi