Perform Apple SDK tests for Mac ports only.

Perform Apple SDK tests for Mac ports only.

Fix after changes of r61832: SDK checks should be done for Mac ports only as
they are useless under the other systems and break configure because sw_vers
is Mac-only and unsurprisingly using it results in errors under the other
systems:

/usr/local/src/wx/HEAD/configure: line 18215: sw_vers: command not found
checking if C compiler works with SDK/version options... configure: error: no.
Try a different SDK

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61837 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775