That works (but I also had to add mk/pthread).

So this means that /usr/pkg/bin/guile and /usr/pkg/lib/libguile.so*
will be linked with pthread.so.  So the following cases should be ok

run /usr/pkg/bin/guile, dynlink .so with pthread

run /usr/pkg/bin/guile, dynlink .so without pthread (since libc is
with pthread to start with?)

run some other program with pthread, and dynlink in libguile

but what about

compile some other program w/o pthreads, and use "guile-config link",
which doesn't have -pthread, to link in guile.

Index: Makefile
===================================================================
RCS file: /NETBSD-CVS/pkgsrc/lang/guile/Makefile,v
retrieving revision 1.52
diff -u -r1.52 Makefile
--- Makefile	3 May 2004 15:58:20 -0000	1.52
+++ Makefile	12 May 2004 22:00:16 -0000
@@ -1,6 +1,7 @@
 # $NetBSD: Makefile,v 1.52 2004/05/03 15:58:20 wiz Exp $
 
 DISTNAME=		guile-1.6.4
+PKGREVISION=		1
 CATEGORIES=		lang
 MASTER_SITES=		${MASTER_SITE_GNU:=guile/}
 
@@ -22,7 +23,12 @@
 TEST_DIRS=		${WRKSRC}
 TEST_TARGET=		check
 
+# Link the native thread so that Guile processes won't die when libpthread.so
+# is dl_open(3)'ed.
+CFLAGS+=		${PTHREAD_CFLAGS}
+
 .include "../../devel/libtool/buildlink3.mk"
 .include "../../devel/ncurses/buildlink3.mk"
 .include "../../devel/readline/buildlink3.mk"
+.include "../../mk/pthread.buildlink3.mk"
 .include "../../mk/bsd.pkg.mk"