Lack of help2man is not detected before compilation

help2man is needed to generate the Superkb man page, but help2man should be detected as a dependency before attempting compilation at all. As of now, it just does it and it fails on make superkb.1:

make superkb.1
make[2]: Entering directory '/home/daniel/repos/superkb'
help2man -n 'Graphical keyboard launcher with on-screen hints' --help-option=-h --version-option=-v -N -i superkb.1.inc ./superkb > superkb.1
/bin/sh: 1: help2man: not found
Makefile:92: recipe for target 'superkb.1' failed