Improved support to build libraries in shared mode

[rhabacker]

The changes from this pull request have been adopted by https://build.opensuse.org/package/show/home:rhabacker:branches:games/librealsense and add support for creating libraries in shared mode, which is required by Linux distributions such as openSUSE.

I did run pr_check.sh and api_check.sh under scripts.

[sysrsbuild]

Can one of the admins verify this patch?

[dg0yt]

• The main library already follows BUILD_SHARED_LIBS.
• I see no effort to handle the necessary export for MSVC.

[rhabacker]

• The main library already follows BUILD_SHARED_LIBS.

There was a spelling error in the commit message, thanks for pointing out.

* I see no effort to handle the necessary  export for MSVC.

The shared build support has been limited to unix like platforms.

[rhabacker]

Support is currently limited to unix-like platforms and is required by packages on build.opensuse.org.

For the creation of packages with shared libraries on Linux distributions such as openSUSE, it should also be noted that third-party packages such as curl and nlohmann_json are included in separate packages, see for example https://build.opensuse.org/projects/hardware/packages/librealsense/files/0001-third-party-use-nlohmann_json-from-system.patch?expand=1

[dg0yt]

Maybe it would be better to not put more topics in a single PR.

[rhabacker]

Maybe it would be better to not put more topics in a single PR.

See #14155