[source] libnl-tiny: define _GNU_SOURCE if not defined

[LEDE Commits]

blogic pushed a commit to source.git, branch master:
https://git.lede-project.org/3c1f20d0bb390f911c3da3334c6d94c19609c8c4

commit 3c1f20d0bb390f911c3da3334c6d94c19609c8c4
Author: Alexandru Ardelean <ardeleanalex at gmail.com>
AuthorDate: Mon Dec 19 17:18:18 2016 +0200

    libnl-tiny: define _GNU_SOURCE if not defined
    
    If _GNU_SOURCE was added as part of a package's TARGET_CFLAGS,
    then compilation would fail for that module (especially if
    warnings get treated as errors).
    
    Signed-off-by: Alexandru Ardelean <ardeleanalex at gmail.com>
---
 package/libs/libnl-tiny/src/include/netlink-local.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/package/libs/libnl-tiny/src/include/netlink-local.h b/package/libs/libnl-tiny/src/include/netlink-local.h
index 330100e..53da8ca 100644
--- a/package/libs/libnl-tiny/src/include/netlink-local.h
+++ b/package/libs/libnl-tiny/src/include/netlink-local.h
@@ -11,7 +11,9 @@
 
 #ifndef NETLINK_LOCAL_H_
 #define NETLINK_LOCAL_H_
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 
 #include <stdio.h>
 #include <errno.h>