Create XP compatible output with VS2012

This pullrequest modifies the "make" files so the output (meridian.exe, blakserv.exe, ...) will be XP compatible even if being built with VS2012.
This is only making any difference for VS 2012 (which by default only targets Vista and higher).

However: This may drop Win2000 support for the output meridian.exe (in case this still is supported at all ??), because it is setting the minimum version explicitly to XP.
This should be tested with VS2008/VS2010 for compatibility.