asinh is part of math.h since C99 times, there's no need to define it
ourselves.
Fixes compilation with Clang: .../marble/src/3rdparty/sgp4/sgp4ext.cpp:210:9: error: 'asinh' is missing exception specification 'throw()' double asinh ^ .../marble/src/3rdparty/sgp4/sgp4ext.h:65:9: note: previous declaration is here double asinh ^