FindTINYXML2.cmake ignores upstream installed tinyxml2Config.cmake

[osrf-migration]

Original report (archived issue) by Silvio Traversaro (Bitbucket: traversaro).

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Since leethomason/tinyxml2#557, tinyxml2 exports its own CMake config file under the CMake package name tinyxml2 . However, ignition-cmake’s FindTINYXML2.cmake CMake scripts ignore this file, and always search for the package using pkg-config or manual search. To correctly find the debug and release version of the library in system without pkg-config, it would be convenient if FindTINYXML2.cmake looked for the tinyxml2 CMake package as the first step, and populated appropriatly the variable and the imported targets that it exports.