GNUInstallDirs: Update FreeBSD "info" destination to share/info

FreeBSD ports commit r484628 (Install texinfo files (GNU info) into
${PREFIX}/share/info, 2018-11-10) changed the "info" destination
from "info" to "share/info".  The commit included a patch to their
distribution of CMake to fix the `GNUInstallDirs` module too.
Apply a similar logic change to our upstream version of the module.

We already made a similar change for GNU/kFreeBSD in commit
v3.13.0-rc2~8^2 (GNUInstallDirs: Don't use BSD info and man paths on
GNU/kFreeBSD, 2018-10-21).

Fixes: #18585
Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win7-windows-release-ninja

passed cmake-hythloth-linux-debug-makefiles+sphinx

passed cmake-hythloth-linux-release-lint-iwyu-tidy

passed cmake-moonshade-osx-debug-ninja

passed cmake-moonshade-osx-release-makefiles

passed cmake-terfin-windows-release-vs2017

passed ghostflow-check-master

passed ghostflow-stager

passed cmake-dash3win7-windows-release-ninja

00:29:11

passed cmake-hythloth-linux-debug-makefiles+sphinx

00:59:28

passed cmake-hythloth-linux-release-lint-iwyu-tidy

00:19:32

passed cmake-moonshade-osx-debug-ninja

00:12:26

passed cmake-moonshade-osx-release-makefiles

00:14:12

passed cmake-terfin-windows-release-vs2017

00:55:31

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager