cmake: fix MinGW build by forcing Vista as minimum target O/S (fixes #44)

This removes the WINXP_SUPPORT option; XP cannot be supported until someone
contributes a version of ThreadSafeInteger that is functional on XP

--HG--
extra : rebase_source : 730e6b66a634c56c9b84f8076db68e45cf26b1fe
cmake: icpc is unable to link cli to shared lib