Use user provided lib/irrlichtmt if available (#11276)

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:

git clone --depth 1 https://github.com/minetest/irrlicht.git lib/irrlichtmt
cmake . -DRUN_IN_PLACE=TRUE