FindIgnOGRE2 finds ogre1.9's libOgreOverlay on Linux if CMAKE_PREFIX_PATH=/usr

[scpeters]

Summary

I originally discovered this issue while researching gazebosim/gz-sensors#62 and found that FindIgnOGRE2 fails to find the correct version of libOgreOverlay in both macOS/homebrew and Linux if CMAKE_PREFIX_PATH is set to /usr (Linux) or /usr/local (macOS homebrew). The issue should be fixed for macOS/homebrew by #125, which is more critical, since the homebrew bottles of ignition-rendering4 are broken without this fix. The issue persists for Linux however.

Background

Both ogre1.9 and ogre2.1 install component libraries named libOgreOverlay. For ogre1.9, an unversioned symbolic link is
installed directly into the lib folder, while in our debian libogre-2.1-dev and brew ogre2.1 packages, an unversioned symbolic
link is installed in lib/OGRE-2.1.

The FindIgnOGRE2 find module uses find_library to find Ogre component libraries with HINTS pointing to the lib/OGRE-2.1 subfolder, which is helpful but limited because the CMAKE_PREFIX_PATH is searched before the HINTS. While a workaround has been added for macOS in #125, it is still broken on Linux if CMAKE_PREFIX_PATH=/usr (see #125 (comment)).

How to reproduce

I've created a debugging branch on release-tools that sets CMAKE_PREFIX_PATH=/usr right before calling cmake .. for Linux builds:

• gazebo-tooling/release-tools@f2ee95f

I ran the ignition_rendering-ci-ign-rendering4-bionic-amd64 build with this release-tools branch and confirmed that many ogre2 tests fail:

• https://build.osrfoundation.org/job/ignition_rendering-ci-ign-rendering4-bionic-amd64/9/