Fixes timezone API definition issues on Windows

GetDynamicTimeZoneInformation, SetDynamicTimeZoneInformation and
GetTimeZoneInformationForYear are provided by the Windows SDK accordingly
with ethe MSDN cocumentation for SDK with versions above 7.1A.

Those functions are incorrectly included by the 7.1A SDK if _WIN32_WINNT
>= 0x0501 instead of _WIN32_WINNT >= 0x0600.

The issue arises when building with an XP compatible toolset (e.g.
v120_xp).
Merge pull request #1892 from alexpilotti/master

Fixes timezone API definition issues on Windows