guile20 does not build

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

Up-to-date netbsd-7, rm -r /usr/pkg and all binary packages removed.
NetBSD  7.1_STABLE NetBSD 7.1_STABLE (XEN3_DOMU.201804190100Z) amd64

=> Checking file-check results for guile20-2.0.14nb2
ERROR: ************************************************************
ERROR: The following files are in the PLIST but not in 
/usr/obj/lang/guile20/work/.destdir/usr/pkg:
ERROR: 
/usr/obj/lang/guile20/work/.destdir/usr/pkg/lib/libguile-2.0.so.22.8.1-gdb.scm
ERROR: ************************************************************
ERROR: The following files are in 
/usr/obj/lang/guile20/work/.destdir/usr/pkg but not in the PLIST:
ERROR: 
/usr/obj/lang/guile20/work/.destdir/usr/pkg/lib/libguile-2.0.a-gdb.scm
*** Error code 1

This is because of the following:

  CCLD     libguile-2.0.la

*** Warning: linker path does not have real file for library -liconv.
*** I have the capability to make that library automatically link in when
*** you link to this library.  But I can only do this if you have a
*** shared version of the library, which you do not appear to have
*** because I did check the linker path looking for a file starting
*** with libiconv and none of the candidates passed a file format test
*** using a regex pattern. Last file checked: 
/usr/obj/lang/guile20/work/.buildlink/lib/libunistring.so
*** The inter-library dependencies that have been dropped here will be
*** automatically added whenever a program is linked with this library
*** or is declared to -dlopen it.

*** Since this library must not contain undefined symbols,
*** because either the platform does not support them or
*** it was explicitly requested with -no-undefined,
*** libtool will only create a static version of it.

As I understand it, it should be using base iconv functionality in libc. 
This was all working, I cannot see what has changed in pkgsrc-2018Q1.

Ideas? I'd like to avoid using PREFER_PKGSRC if I can.

--
Stephen