This is a code generator. It spawns the original version of Fort's asn1/asn1c directory from scratch.
This is only relevant sometimes during Fort's development. If you're a user, or even a casual developer, you don't care.
First, install asn1c. You need a relatively recent commit; the last release is not enough.
git clone https://github.com/vlm/asn1c.git
# This is the one I'm currently using as of 2019-06-14, but the newest available is probably best.
git checkout 88ed3b5cf012918bc1084b606b0624c45e0d2191
cd asn1c
# Dummy installation steps follow. You might be better off reading their INSTALL.md instead.
./configure
make
sudo make install
Then, go to libcmscodec's repository and run autogen.sh.
git clone https://github.com/NICMx/libcmscodec
cd libcmscodec
./autogen.sh
Then manually copy the resulting src/asn1/asn1c directory to Fort.