Fix a warning on Linux caused by _BSD_SOURCE

I got a warning while compiling under Ubuntu 18.04 and gcc 7.4:
"__BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"

Quote from the Linux Programmer's Manual:
"To allow code that requires _BSD_SOURCE in glibc 2.19 and earlier and
_DEFAULT_SOURCE in glibc 2.20 and later to compile without warnings,
define both _BSD_SOURCE and _DEFAULT_SOURCE."
1 file changed