Fix UseUintMax definition used by wxAny for VC6.

Fix UseUintMax definition used by wxAny for VC6.

The old code tried to work around the lack of unsigned __int64 to double
conversion in VC6 by casting from UseUintMax to wxAnyBaseIntType but this was
wrong as this value was -1 when cast to wxAnyBaseIntType (__int64) and so
UseUintMaxF was defined as -1.0.

Use a slightly uglier but simpler work around now: just define the constants
as macros instead of (typed) variables and let the compiler deal with literal
values on its own (which it does correctly).

This fixes the unit test failure: conversion from double to unsigned always
failed when using VC6.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62471 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775