Bug-Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=819886

Quoting gtkdoc-mkpdf.in (and /usr/bin/gtkdoc-mkpdf):
--------------------------
cleanexit() {
    rm -f $module.fo
    exit 1
}
[...]
      echo "dblatex or fop must be installed to use gtkdoc-mkpdf." >&2
      cleanexit 1
    fi
  fi
else
    # not very good output
    # also for xxx-docs.sgml it will produce xxx-docs.pdf
    docbook2pdf -e no-valid "$document"
fi

echo "timestamp" > pdf.stamp
cleanexit 0
--------------------------

So cleanexit() is supposed to take an exitcode as argument but ignores it and exits with 1 instead. Obviously the function definition should read
cleanexit() {
    rm -f $module.fo
    exit $1
}

Whis is exactly how the code looked like in 1.24-2.

This seems to be an unintended change caused by https://git.gnome.org/browse/gtk-doc/commit/?id=12adab6b0a28aec4008c6deed247bf944b5882f2