Only include <endian.h> if _BSD_SOURCE or _DEFAULT_SOURCE
Only include <endian.h> if _BSD_SOURCE or _DEFAULT_SOURCE

glibc <= 2.19 uses _BSD_SOURCE, later uses _DEFAULT_SOURCE. We assume
other platforms are little-endian for now.

steveschnepp/OpenApoc,FranciscoDA/OpenApoc,pmprog/OpenApoc,FranciscoDA/OpenApoc,Istrebitel/OpenApoc,FranciscoDA/OpenApoc,pmprog/OpenApoc,steveschnepp/OpenApoc,Istrebitel/OpenApoc