Define _DEFAULT_SOURCE in addition to _BSD_SOURCE

Fixes many copies of this warning with glibc 2.20.  (See _BSD_SOURCE
in feature_test_macros(7).)

In file included from /usr/include/x86_64-linux-gnu/bits/libc-header-start.h:33:0,
                 from /usr/include/stdint.h:26,
                 from /usr/lib/gcc/x86_64-linux-gnu/7/include/stdint.h:9,
                 from /usr/include/ncursesw/curses.h:63,
                 from owl.h:20,
                 from message.c:1:
/usr/include/features.h:183:3: warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Wcpp]
 # warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
   ^~~~~~~

Signed-off-by: Anders Kaseorg <andersk@mit.edu>