Improve FindIrrlicht.cmake module

Linux distributions prefer to link against a shared version of the Irrlicht
engine instead of using embedded code copies of the same. Search for this
shared version first and use that but fall back to the static version if it
does not exist.

This also fixes minetest#2163

cmake/Modules/FindIrrlicht.cmake

@@ -44,7 +44,7 @@ else()
  /usr/include/irrlicht
  )
 
- FIND_LIBRARY(IRRLICHT_LIBRARY NAMES libIrrlicht.a Irrlicht
+ FIND_LIBRARY(IRRLICHT_LIBRARY NAMES libIrrlicht.so libIrrlicht.a Irrlicht
  PATHS
  /usr/local/lib
  /usr/lib