Build: use GNUInstallDirs CMake module to create directory paths

[fedepell]

Description

Instead of defining manually certain install paths, use the standard (since 3.0) CMake module GNUInstallDirs to derive them. This is cross-platform and offers an automatic integration on distributions customizations (ie. when certain distribution system-wide define a specific place where to install stuff).

See also: https://cmake.org/cmake/help/v3.0/module/GNUInstallDirs.html

The goal is to make the build more modern and software easy to deploy in any distribution.

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• Changes are API compatible.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: CI pass and failing tests are unrelated with the changes.