Also remove the now useless options (like IRRLICHT_INCLUDE_DIR)
and update download instructions, CI and similar.
Co-authored-by: sfan5 <sfan5@live.de>
The recently added ioctl use is reported as a false-positive by valgrind.
I tried moving it to different compilers/versions two times and only
hit further issues that were valgrind's fault.
Also includes a tiny fix.