Add irr::s64 back in (using long long for non VS compilers). If there's any problem with this please inform me, I'd like that to stay in as it makes Irrlicht a lot easier to use in combination with the Clipper library (a boost-licensed library for working with 2d polygons).

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4114 dfc29bdd-3216-0410-991c-e03cc46cb475