backporting gdb build failure fix to binutils 2.30

[Nick Clifton]

Hi Szabolcs,

> i'd like to backport these gdb documentation patches
> into the binutils 2.30 branch, so make install can
> succeed even with an old texinfo:
> 
> https://sourceware.org/ml/gdb-patches/2018-01/msg00259.html
> https://sourceware.org/ml/gdb-patches/2018-01/msg00266.html

Of course - please go ahead and commit these changes.

Cheers
  Nick