Apple/InternalSensors: Fix compilation with FIXED=y

Add fixed casts for assignment of double values to variables of type
fixed
Apple/InternalSensors: Add location data validity checks

According to Apple's CLLocation Class Reference, the longitude and
latitude values are invalid, if horizontalAccuracy is negative, the
altitude value is invalid, if verticalAccuracy is negative, and the
course value is invalid if negative.
Apple/InternalSensors: Set gps.hdop value

Set NMEAInfo gps.hdop value to the horizontalAccuracy received from the
CoreLocation API
doc: document build process for iOS and Mac OS X
Merge branch 'apple' of git://git.xcsoar.org/xcsoar/felix/xcsoar