AP_Notify: disable autobuild of example

this fails due to an AVR linker bug
build: reduced PX4 warnings and avoid rebuilding when possible
Copter: fixed some build warnings
AC_WPNav: fixed some build warnings
AP_Airspeed: fixed a build warning
AP_GPS: fixed a build warning
HAL_PX4: removed an unused variable
AP_Math: added M_PI_F define
PX4: added -Wno-packed

we know that packed data structures are not as efficient