PopupMessage: use constexpr instead of anonymous enum
Screen/ButtonWindow: use constexpr instead of anonymous enum
Android/LogCat: add missing <string> include
Makefile: support libc++ with TARGET=ANDROID
Terrain/RasterMap: add constant "terrain_cache_name"
Terrain/RasterMap: use separate cache files for FIXED=y and FIXED=n

Allows faster switching between different builds.
Airspace: require a non-nullptr AbstractAirspace instance

Convert runtime checks to assertions.