common/osdep: Properly define _POSIX_C_SOURCE on non-Windows
common/osdep: Properly define _POSIX_C_SOURCE on non-Windows

This is required for stricter libc implementations such as musl,
which do not expose fseeko and ftello by default, in order to
strictly follow the spec.

Signed-off-by: Derek Buitenhuis <aff7d637ede26ea82e5226e01b697996cc52e526@gmail.com>

l-smash/l-smash,silverfilain/L-SMASH,maki-rxrz/L-SMASH,silverfilain/L-SMASH,l-smash/l-smash,canbal/l-smash,maki-rxrz/L-SMASH,silverfilain/L-SMASH,l-smash/l-smash,canbal/l-smash