Added SDL_modf() and SDL_modff()

This function is useful for accumulating relative mouse motion if you want to only handle whole pixel movement.
e.g.
static float dx_frac, dy_frac;
float dx, dy;

/* Accumulate new motion with previous sub-pixel motion */
dx = event.motion.xrel + dx_frac;
dy = event.motion.yrel + dy_frac;

/* Split the integral and fractional motion, dx and dy will contain whole pixel deltas */
dx_frac = SDL_modff(dx, &dx);
dy_frac = SDL_modff(dy, &dy);
if (dx != 0.0f || dy != 0.0f) {
    ...
}

include/build_config/SDL_build_config_macos.h

@@ -116,6 +116,7 @@
 #define HAVE_LOG10F 1
 #define HAVE_LROUND 1
 #define HAVE_LROUNDF 1
+#define HAVE_MODF   1
 #define HAVE_POW    1
 #define HAVE_POWF   1
 #define HAVE_ROUND  1