Add cmake installation for spdlog library when it is not locally available or AF_WITH_SPDLOG_HEADER_ONLY is disabled.

Description

• Is this a new feature or a bug fix?: Bug fix
• More detail if necessary to describe all commits in pull request.: adds installation of fetched version of spdlog alongside arrayfire
• Why these changes are necessary.: fixes problems when installing ArrayFire when there is no spdlog library installed in the system and ArrayFire is not build with spdlog header only.
• Potential impact on specific hardware, software or backends.: Installation of spdlog library
• New functions and their functionality.: None
• Can this PR be backported to older versions?: Yes
• Future changes not implemented in this PR.: Possibly adding the ability to build ArrayFire with spdlog statically and not needing to install spdlog

Fixes: #3502

Changes to Users

• No additional options added to the build
• No action required by the user

Checklist

• Rebased on latest master
• Code compiles
• Tests pass
• Functions added to unified API
• Functions documented