OGRE_PLUGIN_PATH definable at build time

[cratliff]

I was attempting the package rviz in a snap and had difficulty because of OGRE_PLUGIN_PATH. The OGRE_PLUGIN_PATH would take a path pointing to a build directory from the package config. This allows OGRE_PLUGIN_PATH to be specified as a cmake arg if the plugin path is known to be different on the system rviz is installed from the one it is built on.

[cratliff]

@wjwwood

Is there any feedback on this PR? If there is additional work to get this merged in I'm happy to do it, but am unsure of what that might be.

The affects a few people who are trying to package rviz in a snap:
https://forum.snapcraft.io/t/encountering-problems-when-packaging-rviz/2659

I would imaging this affects other similar packaging utilities like flatpack too. Being able to package rviz in something like a snap is useful. I'm currently building rviz and custom messages into a snap so I can distribute it to techs and support who need to interact with a robot, but for whom having to checkout and compile the correct version themselves would be onerous. Without this change it is difficult to build a working snap as the OGRE plugin path gets hardcoded to a path not guaranteed to be on the end system.

As an alternative, would making a change that allows rviz to search for the library at runtime be more welcome? I would think using OGRE_PLUGING_PATH when defined, but the current hardcoded value as a default if it is not, is good.