Update OSX SDK checking

We require a min of 10.12 to build these days.
Also reverse the order of the list in configure.ac so that we pick up
newer SDK before older SDK.

Change-Id: Iae25f1971123479801a82c9d30172668a4cfa52a
Reviewed-on: https://gerrit.libreoffice.org/42450Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

README.md

@@ -37,7 +37,7 @@ run and compile LibreOffice, also used by the TDF builds:
  * Build: Cygwin + Visual Studio 2015 Update 3
 * macOS:
  * Runtime: 10.9
- * Build: 10.11 + Xcode 8
+ * Build: 10.12 + Xcode 8
 * Linux:
  * Runtime: RHEL 6 or CentOS 6
  * Build: GCC 4.8.1 or Clang

configure.ac

@@ -2633,7 +2633,7 @@ if test $_os = Darwin -o $_os = iOS; then
     # higher than or equal to the minimum required should be found.
 
     AC_MSG_CHECKING([what Mac OS X SDK to use])
-    for _macosx_sdk in $with_macosx_sdk 10.9 10.10 10.11 10.12 10.13; do
+    for _macosx_sdk in $with_macosx_sdk 10.13 10.12; do
         MACOSX_SDK_PATH=`xcrun --sdk macosx${_macosx_sdk} --show-sdk-path 2> /dev/null`
         if test -d "$MACOSX_SDK_PATH"; then
             with_macosx_sdk="${_macosx_sdk}"