Fixed build VS2013

C99 library support added in Visual Studio causes compiler error
C2084 due to fmin/fmax from bgfx\tools\geometryc\math.h and
warning C28251 bgfx\examples\common\fpumath.h due to fminf/fmaxf.

This commit works around these issues by adding new define
BX_COMPILER_MSVC_VER to bx\platform.h