This worked for me to run on OpenBSD.  Update package versions as needed:

# pkg_add autoconf-2.71 automake-1.16.5 gettext-tools-0.21.1 gtk3mm-3.24.7
# export AUTOMAKE_VERSION=1.16 AUTOCONF_VERSION=2.71 MAKE=gmake
# ./autogen.sh
# ./configure
# gmake
# ./src/xnec2c examples/airplane.nec
