Use gtar to extract

On NetBSD 6, the system pax fails to extract the tarball.   Until this
is understood, just use gtar.

net/syncthing/Makefile

@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.19 2016/06/01 18:46:00 gdt Exp $
+# $NetBSD: Makefile,v 1.20 2016/06/01 18:51:59 gdt Exp $
 
 # Upstream regularly breaks protocol compatibility.  While users of
 # pkgsrc syncthing on multiple systems can update synchronously, an
@@ -24,6 +24,9 @@ HOMEPAGE=	http://syncthing.net/
 MAINTAINER=	gdt@NetBSD.org
 LICENSE=	mpl-2.0
 
+# System pax fails on NetBSD 6.  Just use gtar until this is understood.
+EXTRACT_USING=		gtar
+
 GITHUB_TAG=	v${PKGVERSION_NOREV}
 
 BUILD_DEPENDS+=		go-[0-9]*:../../lang/go