Use _get_timezone() function instead of _timezone with MSVC8+.

Use _get_timezone() function instead of _timezone with MSVC8+.

While some (but not all) versions of VC8 CRT still define _timezone variable,
it is deprecated and shouldn't be used and referencing it can result in
linking problems if it pulls in static CRT.

Just use _get_timezone() function instead for the VC versions that support it
(as was already done in r54417 for VC8 in 2.8 branch).

Closes #4691.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65994 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775