Use user provided lib/irrlichtmt if available (#11276)
Use user provided lib/irrlichtmt if available
Make it possible for a user to provide the IrrlichtMt dependency as
subdirectory at `lib/irrlichtmt`.
The subdirectory is added with the `EXCLUDE_FROM_ALL` flag to prevent
`libirrlichtmt.a` or other header files to be installed.
This enables the user to do the following to satisfy the IrrlichtMt
dependency: