changed isnan checks and cmake compiler condition for apple machines

src/lib/mathlib/math/filter/LowPassFilter2p.cpp

@@ -74,7 +74,7 @@ float LowPassFilter2p::apply(float sample)
 
     // do the filtering
     float delay_element_0 = sample - _delay_element_1 * _a1 - _delay_element_2 * _a2;
-    if (isnan(delay_element_0) || isinf(delay_element_0)) {
+    if (!PX4_ISFINITE(delay_element_0)) {
         // don't allow bad values to propagate via the filter
         delay_element_0 = sample;
     }