[Gazebo9][ign-cmake0] FindOgre.cmake should define OGRE variable names more consistently with OgreConfig.cmake

[osrf-migration]

Original report (archived issue) by Sean Yen (Bitbucket: seanyen-msft).

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This issue was originally reported https://github.com/ms-iot/ROSOnWindows/issues/48.

After some investigation, I found the OGRE variables defined in FindOgre.cmake seems not to be using the same naming convention to OgreConfig.cmake and one mitigation is proposed here: https://bitbucket.org/osrf/gazebo/pull-requests/3090

References:

• OgreConfig.cmake.in in Ogre side: https://github.com/OGRECave/ogre/blob/v1.10.12/CMake/Templates/OGREConfig.cmake.in#L54
• FindOgre.cmake in ign-cmake side: https://github.com/ignitionrobotics/ign-cmake/blob/4d0553012f377a6ce70a22ef3df8d9d30d1ba4e4/cmake/FindOGRE.cmake#L70