Previously __WIN64__ was only defined for 64 bit builds with MSVC, which
resulted in many problems when using 64 bit Cygwin compiler.
Also don't use MSVC-specific __int64 but our wxInt64 for WX{L,W}PARAM and
WXLRESULT definitions in 64 bit builds.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76511 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775