[CImgPlugin] Use sofa cmake command to create proper package

[thomas-lemaire]

With this PR you can find_package(CImgPlugin) in an other cmake project. Quite useful since this plugin is indeed part of the core

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This PR:

• builds with SUCCESS for all platforms on the CI.
• does not generate new warnings.
• does not generate new unit test failures.
• does not generate new scene test failures.
• does not break API compatibility.
• is more than 1 week old (or has fast-merge label).

Reviewers will merge only if all these checks are true.

[damienmarchal]

+1

To CmakeLists.txt guys,

just a small question I have

• what is this for ?
  if(NOT TARGET CImgPlugin)
  include("${CMAKE_CURRENT_LIST_DIR}/CImgPluginTargets.cmake")
  endif()

• shouldn't we also export something like
  CImgPlugin_INCLUDE_DIR ?

To make the plugin relocatable ?

[guparan]

I need these changes for v17.12 so I fast-merge them.

@damienmarchal : @fredroy should know the answer to your questions.