It was quite easy to get openfortivpn going on the latest version of Clear Linux. (the command line client at least)
$ sudo swupd bundle-add devpkg-openssl
$ sudo swupd bundle-add devpkg-systemd
Download the sources:
https://codeload.github.com/adrienverge/openfortivpn/tar.gz/refs/tags/v1.17.1
Unzip, enter the new dir and run:
$ ./autogen.sh
$ ./configure --prefix=/usr/local --sysconfdir=/etc
$ make
$ sudo make install
At this point i could run the binary, but it gives me an error that /dev/ppp is not running.
So I ran:
$ sudo /dev/ppp
This resulted in an error, but gave the suggestion to perform this command. (Note; this one might be specific for my system).
$ sudo mknod /dev/ppp c 108 0
And now it should work. At least for me it did 