include/u.h: define _DEFAULT_SOURCE for new glibc

glibc devs have apparently decided _BSD_SOURCE will be
deprecated on Linux, and issue a preprocessor warning if
declaring _BSD_SOURCE without _DEFAULT_SOURCE.

https://sourceware.org/glibc/wiki/Release/2.20

Fixes #8397.

LGTM=iant
R=dave, gobot, iant
CC=golang-codereviews
https://golang.org/cl/112530043

include/u.h

@@ -40,6 +40,7 @@ extern "C" {
 #endif
 #define _BSD_SOURCE 1
 #define _NETBSD_SOURCE 1	/* NetBSD */
+#define _DEFAULT_SOURCE 1	/* glibc > 2.19 */
 #define _SVID_SOURCE 1
 #if !defined(__APPLE__) && !defined(__OpenBSD__)
 #	define _XOPEN_SOURCE 1000