gdb build broken in libctf with old makeinfo

[Tom de Vries]

Hi,

I tried to build gdb on an older distro, with an old version of
makeinfo, version 4.13.

While building the libctf bit configure informed me that makeinfo is too
old and "Info documentation will not be build".  Subsequently, the build
broke on building precisely that documentation.

Reverting to before commit 1eaa86a6d5b "libctf: merge doc subdir up a
level" fixes the build.

Thanks,
- Tom