Drop support for Windows XP
Windows XP hit its end-of-life today (April 8, 2014).
We should identify and drop support code for Windows XP. This is mainly going to be a matter of identifying cases where we use LoadLibrary and GetProcAddress to find always-present-functions in always-present DLLs, and looking for opportunities to move from old busted APIs to fresh new ones.
I'm making this a separate ticket from #11444 (moved) (removing support from pre-XP versions) since the timing on the two can be argued to be separate. Nonetheless, if we agree to do both at once, that might be clever.
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