install destination paths do not correspond to iRODS paths

[d-w-moore]

@SwooshyCueb :
Using: cmake 3.11.4-0 ; ubuntu 18.04 ; building the repo on branch 4-2-stable
But results were same on Centos 7, and on master branch.

Building and installing from source , neither of these two alternatives of installing CMake-built TARGETs and FILES from this repo end up placing the targets/files into the customary irods paths (/usr/lib/irods and /var/lib/irods)
(a) direct: make install
(b) package: make package, followed by installing the generated .deb or .rpm package

This is because non-empty extra path segments are being prepended to the desired destination path, making for example /usr/lib/irods into /usr/usr/lib/irods

Currently, building an unmodified tip of 4-2-stable of this repo against iRODS 4.2.8 results in these (undesired) path segments being prepended to /usr/lib/irods and /var/lib/irods in each target/file:

(a) direct: /usr/local
(b) package: /usr

But if we place the line

set(CMAKE_INSTALL_PREFIX "/" CACHE STRING "Install root path. Change from \"/\" for run-in-place installations." FORCE)

directly after the project line in CMakeLists.txt to try and enull those prepended paths, we get:

(a) direct: (empty = successful install) , ie the "/" CMAKE_INSTALL PATH worked!
(b) package: /usr

So, effectively. only the (a) direct install method is being corrected by setting the CMAKE_INSTALL_PREFIX cache variable to "/" , whereas the package install logic is following some other, yet unknown cue.)

[d-w-moore]

In 4.2.8.0 we actually did set the CPACK_PACKAGING_INSTALL_PREFIX to / which causes non-absolute install paths to be placed at the root filesystem. However in 4-2-stable we no longer set this cache variable, and so non-absolute install paths such as IRODS_HOME_DIRECTORY (= var/lib/irods) are receiving the unwanted "/usr" from the default cpack generator

[d-w-moore]

@SwooshyCueb I see this was done as part of the move to using GnuInstallDirs ... not sure how one would implement a compromise between the two approaches. Thoughts?

[SwooshyCueb]

The movie to using GNUInstallDirs isn't entirely backwards compatible. If you want to build against 4.2.8 and not stable-4-2, I think you will have to pass some definitions to CMake on the command line.
I think you will need -DCMAKE_INSTALL_PREFIX=/ and potentially also -DCMAKE_INSTALL_LIBDIR=usr/lib.

If building the HEAD of 4-2-stable in our plugin repositories against the previous release is something we want to support, we can re-introduce the manual set of CMAKE_INSTALL_PREFIX for 4-2-stable in the plugin repositories, but I would recommend against doing this for master. Either way, it should not be placed before the call to find_pacakge(IRODS.