Support GN build on host

Change-Id: I072566ccf280f4ad346de67a10016cc44683814e
diff --git a/BUILD.gn b/BUILD.gn
index bad66ee..0d2e65e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -51,6 +51,9 @@
     "HAVE_CONFIG_H",
     "_REENTRANT",
   ]
+  if (is_linux) {
+    defines += [ "_GNU_SOURCE" ]
+  }
 }
 
 shared_library("libxml2") {