C++ libtool build error?

[Simon Josefsson]

Recently gnulib added some self-tests written in C++ for otherwise
C-only modules.  When imported into libidn (otherwise a strictly C
library), this leads to an error message:

/bin/bash ../libtool --preserve-dup-deps  --mode=link g++     -o 
test-fcntl-h-c++ test-fcntl-h-c++.o libtests.a ../gl/libgnu.la libtests.a    
libtool: link: unable to infer tagged configuration
libtool: link: specify a tag with `--tag'
make[4]: *** [test-fcntl-h-c++] Error 1
make[4]: Leaving directory `/home/jas/src/libidn/gltests'

What do I have to do to make this work?  Is it something that
gnulib-tool should have done for me?

/Simon