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

The `GNUInstallDirs` module has different defaults on BSD platforms.
Update the test expected output to account for the difference.

Fixes: #16887

Teach the module to handle SYSCONFDIR and LOCALSTATEDIR properly if
CMAKE_INSTALL_PREFIX is set to `/` or `/usr` -- i.e. as expected by GNU
Coding Standard (i.e. set SYSCONFDIR to `/etc` and `LOCALSTATEDIR` to
`/var`).  Also if CMAKE_INSTALL_PREFIX is set to /opt/pkg, `SYSCONFDIR`
must be set to `/etc/opt/pkg` and `LOCALSTATEDIR` to `/var/opt/pkg`
according to FHS.

Add a RunCMake.GNUInstallDirs test with cases covering various install
prefixes.  Hard-code the architecture information.  Tolerate all
platform-specific LIBDIR values.

Currently the root prefix is not handled well, but verify the current
behavior in the test anyway.  This can be addressed with a future
change.
Inspired-by: Alex Turbov <i.zaufi@gmail.com>