golang-cross: do_compile fails cc1: all warnings being treated as errors

glibc 2.20 deprecates _BSD_SOURCE and _SVID_SOURCE and emits an error
message.  From patch 16632:
	libc [PATCH] BZ #16632: Disable _SVID_SOURCE/_BSD_SOURCE warning
	if _DEFAULT_SOURCE is defined

Since we also need to support glibc before 2.20, from the release notes
for glibc 2.20, the recommended fix is to define _DEFAULT_SOURCE

(fixed upstream)
https://groups.google.com/forum/#!topic/golang-codereviews/S4TARFCxu2k

Signed-off-by: Amy Fong <amy.fong@windriver.com>
---
 include/u.h |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/include/u.h
+++ b/include/u.h
@@ -38,6 +38,16 @@
 #		define __MAKECONTEXT_V2_SOURCE 1
 #	endif
 #endif
+/**
+ * in glibc >= 2.20, _BSD_SOURCE and _SVID_SOURCE causes warning
+ * messages if _DEFAULT_SOURCE is not defined.
+ *
+ * From glibc 2.20 release notes, since this application needs _BSD_SOURCE
+ * and/or _SVID_SOURCE and we must support glibc < 2.19 and
+ * glibc >= 2.20, then define all 3 (_DEFAULT_SOURCE, _BSD_SOURCE,
+ * and _SVID_SOURCE) unconditionally
+ */
+#define _DEFAULT_SOURCE 1
 #define _BSD_SOURCE 1
 #define _NETBSD_SOURCE 1	/* NetBSD */
 #define _SVID_SOURCE 1