Fix VsnprintfTestCase for recent MinGW versions.

Fix VsnprintfTestCase for recent MinGW versions.

MinGW now uses its own printf() implementation which handles %p differently
from the VC CRT one and uses %8x instead of %8X for it. Compare the results of
wxPrintf("%p") case-insensitively to let the test pass in any case.

Also introduce a USING_VC_CRT macro instead of testing for
__USE_MINGW_ANSI_STDIO in two different places.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65738 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775