/bin/sh: line 1: docbook2x-texi: command not found
This is the docbook to texi converter from the docbook2X package (which has not been updated since 2007).
For Red Hat and Fedora, the EPEL package repo provides docbook2X. However, the name of the script is changed because there is a newer docbook package. So, in the git source directory, edit the file Documentation/Makefile and change one line:
DOCBOOK2X_TEXI = db2x_docbook2texi