Build selective portions of gnu/usr.bin/texinfo as part of build-tools to
ensure that building on a host without makeinfo (i.e. a host where
make delete-old -DWITHOUT_INFO was run), then building with MK_INFO == yes
doesn't manifest in build errors when building info pages

This manifested itself like the following when I was build testing an MFC
change on stable/10:

  makeinfo --no-split -I /usr/src/gnu/lib/libregex/doc -I /usr/src/gnu/lib/libregex/doc regex.texi  -o regex.info
  makeinfo: not found
  *** [regex.info] Error code 127

make[6]: stopped in /usr/src/gnu/lib/libregex/doc
1 error

Tested on a head VM without makeinfo installed and by building with MK_INFO=yes

MFC after: 1 week
Update the text for building texinfo with build-tools to reflect the fact
that make install is being called as well

MFC after: 1 week
X-MFC with: r276052