Bug#865180: On Big Endian support

[Kai Pastor, DG0YT]

Relevant upstream changes for Big Endian builds are

aed9a2d7d20d019c8cf5dff0d8e5edeee2b4d2d8
14a683b07b7657f766b84c6c67f9080bef8af7c5
Cf. 
https://github.com/OpenOrienteering/mapper/commits/14a683b07b7657f766b84c6c67f9080bef8af7c5/CMakeLists.txt