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) {
    ...
}

CMakeLists.txt

@@ -962,8 +962,8 @@ if(SDL_LIBC)
             _stricmp _strnicmp sscanf
             acos acosf asin asinf atan atanf atan2 atan2f ceil ceilf
             copysign copysignf cos cosf exp expf fabs fabsf floor floorf fmod fmodf
-            log logf log10 log10f lround lroundf pow powf round roundf scalbn scalbnf
-            sin sinf sqrt sqrtf tan tanf trunc truncf)
+            log logf log10 log10f lround lroundf modf modff pow powf round roundf
+            scalbn scalbnf sin sinf sqrt sqrtf tan tanf trunc truncf)
       string(TOUPPER ${_FN} _UPPER)
       set(HAVE_${_UPPER} 1)
     endforeach()