I was in a hurry yesterday, so I have accidentally sent the wrong log file... :-/ Today, I looked into this more:
* I am able to create documentation if I normally use 'make -C doc/ pdf' or 'cd doc && make pdf && cd ..' for gawk-4.1.4 in my Fedora 24.
* The problem occurs when I try to build it inside mock  for Fedora 24 (and higher), I'm trying to do rebase for F26.
* If I try to build it for F23 in mock, it passes. IOW, using newer versions of 'texi2dvi' inside mock (for F24+) results in compilation fail.
* When it fails, it produces only partial documentation, with Index and everything behind it missing.
* When I use the 'texi2pdf' instead of 'make pdf' in the specfile, I'm able to see at which line this occurs.
* I've used git-bisect to find a commit where the build starts to fail, it's this commit:
[upstream||gawk] ~ [3e762b6 $] # git last
* commit 3e762b6f62061974d152dd251f3e83cacca073e3 (HEAD, refs/bisect/bad)
| Author: Arnold D. Robbins <address@hidden>
| Date: 12 months ago
| Update infrastructure files.
* This helped me narrow it to file causing the issue: doc/texinfo.tex (using older version of texinfo.tex with the first failing commit allows build to pass).
* Reading through changes in the texinfo.tex, there was some bigger update to how Index is being produced, if I'm not mistaken.
* Running the build with refs/bisect/bad commit and with 'texi2pdf' shows me, that the compilation of PDF stops at line number 38144 of gawk.texi (which has @cindex right above it).
Unfortunately, I'm not so good with TeX to be able to pinpoint the problem more closely, but if you need anything more from me, I can help. :) I can also provide you with the srpm if you would like to build it / try it yourself.