Bump Xcode baseline to 9.3

...as discussed at
<https://lists.freedesktop.org/archives/libreoffice/2018-November/081435.html>
"minutes of ESC call ..."

Change-Id: I47e23d222b4b27256483dfd7de915dd99ae81317
Reviewed-on: https://gerrit.libreoffice.org/63878
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

README.md

@@ -37,7 +37,7 @@ run and compile LibreOffice, also used by the TDF builds:
     * Build: Cygwin + Visual Studio 2017
 * macOS:
     * Runtime: 10.9
-    * Build: 10.12 + Xcode 8
+    * Build: 10.12 + Xcode 9.3
 * Linux:
     * Runtime: RHEL 6 or CentOS 6
     * Build: GCC 4.8.1 or Clang

configure.ac

@@ -2773,6 +2773,16 @@ if test $_os = Darwin -o $_os = iOS; then
     FRAMEWORKSHOME="$MACOSX_SDK_PATH/System/Library/Frameworks"
     MACOSX_DEPLOYMENT_TARGET="$with_macosx_version_min_required"
 
+    AC_MSG_CHECKING([whether Xcode is new enough])
+    my_xcode_ver1=$(xcrun xcodebuild -version | head -n 1)
+    my_xcode_ver2=${my_xcode_ver1#Xcode }
+    my_xcode_ver3=$(printf %s "$my_xcode_ver2" | $AWK -F. '{ print $1*100+($2<100?$2:99) }')
+    if test "$my_xcode_ver3" -ge 903; then
+        AC_MSG_RESULT([yes ($my_xcode_ver2)])
+    else
+        AC_MSG_ERROR(["$my_xcode_ver1" is too old or unrecognized, must be at least Xcode 9.3])
+    fi
+
     case "$with_macosx_version_min_required" in
     10.9)
         MAC_OS_X_VERSION_MIN_REQUIRED="1090"