bind-tools Build Error

Hi all.

For the past 2 weeks, I've been having an issue on only 4 of our name 
servers failing to upgrade (or rebuild from scratch) bind-tools:

*****

ld: error: ../../lib/isc/.libs/libisc.so: undefined reference to 
UCNV_FROM_U_CALLBACK_STOP [--no-allow-shlib-undefined]
ld: error: ../../lib/isc/.libs/libisc.so: undefined reference to 
UCNV_TO_U_CALLBACK_STOP [--no-allow-shlib-undefined]
ld: error: ../../lib/isc/.libs/libisc.so: undefined reference to 
ucnv_close [--no-allow-shlib-undefined]
ld: error: ../../lib/isc/.libs/libisc.so: undefined reference to 
ucnv_convertEx [--no-allow-shlib-undefined]
ld: error: ../../lib/isc/.libs/libisc.so: undefined reference to 
ucnv_open [--no-allow-shlib-undefined]
ld: error: ../../lib/isc/.libs/libisc.so: undefined reference to 
ucnv_setFromUCallBack [--no-allow-shlib-undefined]
ld: error: ../../lib/isc/.libs/libisc.so: undefined reference to 
ucnv_setToUCallBack [--no-allow-shlib-undefined]
cc: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[5]: *** [Makefile:623: cfg_test] Error 1
gmake[5]: Leaving directory 
'/usr/ports/dns/bind-tools/work/bind-9.18.2/doc/misc'
gmake[4]: *** [Makefile:450: all-recursive] Error 1
gmake[4]: Leaving directory '/usr/ports/dns/bind-tools/work/bind-9.18.2/doc'
gmake[3]: *** [Makefile:571: all-recursive] Error 1
gmake[3]: Leaving directory '/usr/ports/dns/bind-tools/work/bind-9.18.2'
gmake[2]: *** [Makefile:498: all] Error 2
gmake[2]: Leaving directory '/usr/ports/dns/bind-tools/work/bind-9.18.2'
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/dns/bind-tools
*** Error code 1

Stop.
make: stopped in /usr/ports/dns/bind-tools

*****

I gave it a week to see if this is a build issue, and that the new Ports 
tree can fix whatever it is. However, the issue persists.

System is 13.0-RELEASE-p11.

Just to note that there is no issue building BIND itself on these 4 boxes.

All other boxes are able to build bind-tools with no issue.

Has anyone else seen this, or knows what the issue might be?

Mark.