stop manually defining physical constants

src/modules/systemlib/CMakeLists.txt

@@ -32,7 +32,7 @@
 ############################################################################
 
 set(SRCS
-	airspeed.c
+	airspeed.cpp
 	battery.cpp
 	board_serial.c
 	bson/tinybson.c

src/modules/systemlib/airspeed.cpp

@@ -236,7 +236,7 @@ float calc_true_airspeed(float total_pressure, float static_pressure, float temp
 {
 	float density = get_air_density(static_pressure, temperature_celsius);
 
-	if (density < 0.0001f || !isfinite(density)) {
+	if (density < 0.0001f || !PX4_ISFINITE(density)) {
 		density = CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C;
 	}