Add the feature test macro which is required for building with the
musl toolchain.

The feature test macro _XOPEN_SOURCE = 600 provides the XSI-compliant
version of strerror_r().
Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>