C++ has undefined behavior for identifiers starting with __ or with _ followed by an uppercase letter.
We still have many more (in IrrCompileConfig.h and in all header-guards), will likely replace those later as well.
As a workaround for users which might use irrlicht defines in their code, I've added the header irrLegacyDefines.h
Including that allows to continue using old defines for a while - or make it easier to have code which compiles
with old and new Irrlicht library versions.
git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6251 dfc29bdd-3216-0410-991c-e03cc46cb475
