Inconsistencies with install() and relative path to GNUInstallDirs

CMake implementation of install() seems inconsistent with what the documentation recommends.

Current documentation

The documentation of install() specifies that:

DESTINATION <dir>

Specify the directory on disk to which a file will be installed. <dir> should be a relative path. An absolute path is allowed, but not recommended.

[…]

To make packages compliant with distribution filesystem layout policies, if projects must specify a DESTINATION, it is strongly recommended that they use a path that begins with the appropriate relative GNUInstallDirs variable.

and the documentation of GNUInstallDirs specifies that:

CMAKE_INSTALL_<dir>

Destination for files of a given type. This value may be passed to the DESTINATION options of install() commands for the corresponding file type. It should be a path relative to the installation prefix so that it can be converted to an absolute path in a relocatable way.

While absolute paths are allowed, they are not recommended as they do not work with the cmake --install command's --prefix option, or with the cpack installer generators. In particular, there is no need to make paths absolute by prepending CMAKE_INSTALL_PREFIX; this prefix is used by default if the DESTINATION is a relative path.

GNUInstallDirs with different CMAKE_INSTALL_PREFIX

Here is a table of the install directories computed by GNUInstallDirs depending on CMAKE_INSTALL_PREFIX on a Debian machine:

Prefix/Dir	/usr	/usr/local	/home/foo
BINDIR	relative=bin, full=/usr/bin	relative=bin, full=/usr/local/bin	relative=bin, full=/home/foo/bin
SBINDIR	relative=sbin, full=/usr/sbin	relative=sbin, full=/usr/local/sbin	relative=sbin, full=/home/foo/sbin
LIBEXECDIR	relative=libexec, full=/usr/libexec	relative=libexec, full=/usr/local/libexec	relative=libexec, full=/home/foo/libexec
SYSCONFDIR	relative=etc, full=/etc	relative=etc, full=/usr/local/etc	relative=etc, full=/home/foo/etc
SHAREDSTATEDIR	relative=com, full=/usr/com	relative=com, full=/usr/local/com	relative=com, full=/home/foo/com
LOCALSTATEDIR	relative=var, full=/var	relative=var, full=/usr/local/var	relative=var, full=/home/foo/var
RUNSTATEDIR	relative=var/run, full=/var/run	relative=var/run, full=/usr/local/var/run	relative=var/run, full=/home/foo/var/run
LIBDIR	relative=lib/x86_64-linux-gnu, full=/usr/lib/x86_64-linux-gnu	relative=lib, full=/usr/local/lib	relative=lib, full=/home/foo/lib
INCLUDEDIR	relative=include, full=/usr/include	relative=include, full=/usr/local/include	relative=include, full=/home/foo/include
OLDINCLUDEDIR	relative=/usr/include, full=/usr/include	relative=/usr/include, full=/usr/include	relative=/usr/include, full=/usr/include
DATAROOTDIR	relative=share, full=/usr/share	relative=share, full=/usr/local/share	relative=share, full=/home/foo/share
DATADIR	relative=share, full=/usr/share	relative=share, full=/usr/local/share	relative=share, full=/home/foo/share
INFODIR	relative=share/info, full=/usr/share/info	relative=share/info, full=/usr/local/share/info	relative=share/info, full=/home/foo/share/info
LOCALEDIR	relative=share/locale, full=/usr/share/locale	relative=share/locale, full=/usr/local/share/locale	relative=share/locale, full=/home/foo/share/locale
MANDIR	relative=share/man, full=/usr/share/man	relative=share/man, full=/usr/local/share/man	relative=share/man, full=/home/foo/share/man
DOCDIR	relative=share/doc/test, full=/usr/share/doc/test	relative=share/doc/test, full=/usr/local/share/doc/test	relative=share/doc/test, full=/home/foo/share/doc/test

Problem

The problem is that when the prefix is /usr, it is not possible to use a relative path to install in SYSCONFDIR.

install(
  FILES foo.txt
  TYPE SYSCONF # or DESTINATION ${CMAKE_INSTALL_SYSCONFDIR}
)

It will install in /usr/etc instead of /etc. It means that we have to use CMAKE_INSTALL_FULL_<dir> for this case which:

1. is contrary to the documentation
2. prevents usage of the --prefix option

Related to https://discourse.cmake.org/t/gnuinstalldir-differences-in-prefix-paths-and-inconsistency-with-documentation/5102.