Extract compiler-specific macro definitions in a new wx/compiler.h.

Extract compiler-specific macro definitions in a new wx/compiler.h.

This solves the problem with wx/defs.h -> wx/platform.h -> wx/setup.h which
resolves to msvc/wx/setup.h -> wx/version.h -> wx/cpp.h include path which
resulted in __VISUALC__ not being defined in wx/cpp.h.

This problem was not new but went unnoticed for a long time and was only
discovered when wxCHECK_VISUALC_VERSION() started being used in wx/cpp.h too
as now the compiler started warning about wrong #if syntax due to it being
undefined.

Putting the compiler-specific definitions in a separate file allows this file
to be included from wx/cpp.h to ensure that these symbols are always defined
in it and also makes things a little better organized.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74496 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775