Rename Assert.h to Assert2.h.

Prevents issues where it caused #include <assert.h> in third party headers to not include the correct header.
CMakeLists.txt: Link with 64 bit libraries instead of 32 bit.
SDL2_gfxPrimitives: Fix redefining function which was added to VS2013 when compiling with VS.
main.cpp: Remove unused variables and function.
CMakeLists.txt: Add Box2D dependency.
Assert2.h: Replace VS-specific pragmas with macros.
CMakeLists.txt: Fix box2d module name.