guile fails to build without makeinfo:

make[4]: Entering directory
'/nvmedata/autobuild/instance-7/output-1/build/host-guile-2.0.14/doc/ref'
  MAKEINFO guile.info
/nvmedata/autobuild/instance-7/output-1/build/host-guile-2.0.14/build-aux/missing:
line 81: makeinfo: command not found
WARNING: 'makeinfo' is missing on your system.
         You should only need it if you modified a '.texi' file, or
         any other file indirectly affecting the aspect of the manual.
         You might want to install the Texinfo package:
         <http://www.gnu.org/software/texinfo/>
         The spurious makeinfo call might also be the consequence of
         using a buggy 'make' (AIX, DU, IRIX), in which case you might
         want to install GNU make:
         <http://www.gnu.org/software/make/>

To fix this build failure, the attached patch don't build the doc
subdir if makeinfo is not available.

Fixes:
 - http://autobuild.buildroot.org/results/9605aac6f760bfff190d0ab95fa50f65486ffe90