Test for sw_vers availability before calling it in configure.

There is no sw_vers when cross-compiling, just assume 10.4 system in this
case and point the user to --with-macosx-version-min which can be used to
override this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62869 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

configure.in

@@ -1205,7 +1205,15 @@ dnl We need to quote the next line where we don't need macros and do need [] in
         wxUSE_MACOSX_VERSION_MIN=
     fi
 elif test "x$wxUSE_MACOSX_VERSION_MIN" = "x"; then
-    OSX_VERSION=`sw_vers -productVersion | grep 10.[[0-9]]`
+    AC_CHECK_PROGS(OSX_SW_VERS, sw_vers)
+    if test "$OSX_SW_VERS" != ""; then
+        OSX_VERSION=`sw_vers -productVersion | grep 10.[[0-9]]`
+    else
+        dnl can't determine it (happens e.g. when cross-compiling) so use a
+        dnl conservative default
+        AC_MSG_WARN([Assuming OS X 10.4, use --with-macosx-version-min to override.])
+        OSX_VERSION="10.4"
+    fi
     if test "$wxUSE_OSX_CARBON" = 1 -o "x$OSX_VERSION" = "x10.4"; then
         # otherwise configure stops on leopard for universal_binary
         wxUSE_MACOSX_VERSION_MIN=10.4