Isok uses the PGXN.org tools for building and distribution, which in turn uses parts of the PostgreSQL GNU make-based build system. So GNU make is required. In addition, the Isok documentation is done with the DocBook, as is PostgreSQL's, so the tooling required to build the documentation is the same as PostgreSQL, although Isok currently generates XHTML so may require a slightly different set of DTDs.
Aside from these requirements, the following additional tools are needed:
The DocBook to LaTeX to PDF, etc., converter
The GNU awk implementation
The macro pre-processor used by GNU autoconf, etc.
The command-line web browser
The XML linter
The archive and compression tool
DBLatex also requires the installation of various TeX and LaTeX tooling, which your O/S's package manager is likely to install as a dependency.
When working with DocBook, the book DocBook XSL: The Complete Guide from Sagehill.net may also be useful.