In this directory, run:

autoconf
./configure
make

On success, you should find the documentation in the subdirectory happy/.

Running ./configure might report:

checking for DocBook XSL stylesheet directory... no
configure: WARNING: cannot find DocBook XSL stylesheets, you will not be able to build the documentation

Extending the list FP_DIR_DOCBOOK_XSL in file configure.ac might help. E.g., on Mac OS X with homebrew-installed docbook, the path

/usr/local/Cellar/docbook-xsl/*/docbook-xsl

worked. Inside this directory (pattern), configure looks for a file /html/docbook.xsl, see aclocal.m4.