We need to define _XOPEN_SOURCE and __EXTENSIONS__ macros in order to get
CMSG_ and TIOCWIN macros defined. But then _POSIX_C_SOURCE gets defined, which
is (incorrectly) used as an indicator for existence of posix_memalign() in osdep.c.
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>