Their build tooling seems very sparse. In particular, it does not
support DESTDIR or PREFIX variables. However, that might be OK in this
case, as it appears to only produce and install a single artifact: a
binary called fzf.
If I were in your position, I would run 'make' and then manually place
the fzf program in a location where the shell's PATH variable setting
will allow the fzf program to be found. Likely either /usr/local/bin
(if you want it to be available to everyone on the system) or ~/bin (if
you only want to make it available to your own login without affecting
others who might be using the system-provided binary).
Either way, the simplicity of the tool (in terms of it being a single
binary artifact that is deployed) makes it unlikely that you would
encounter any issues in doing this.
Regards,
-Roberto