Avoid a warning if _BSD_SOURCE is already defined.

As _BSD_SOURCE has been deprecated since glibc 2.20, also add
_DEFAULT_SOURCE to the definitions. Both can be any value as long
as they're defined, so in order to save two lines, just #undef
them instead of checking with #ifdef/#endif.

src/file.cpp

@@ -278,7 +278,10 @@ done:
 	}
 }
 # else
-#  define _BSD_SOURCE
+#  undef _BSD_SOURCE
+#  define _BSD_SOURCE // deprecated since glibc 2.20
+#  undef _DEFAULT_SOURCE
+#  define _DEFAULT_SOURCE
 #  include <sys/uio.h>
 # endif
 #endif