Take advantage of IrrlichtMt CMake target (#11287)

With the CMake changes to IrrlichtMt, it's now possible to use a target for IrrlichtMt.
Besides greatly improving the ease of setting up IrrlichtMt for users building the client, it removes the need for Minetest's CMake to include transitive dependencies such as image libraries, cleaning it up a tiny bit. The PR works by finding the IrrlichtMt package and linking to the target it provides. If the package isn't found and it isn't building the client, it will still fall back to using just the headers of old Irrlicht or IrrlichtMt.

1 files changed, 3 insertions, 0 deletions

diff --git a/README.md b/README.md
index 0cd134f27..1774d1ea3 100644
--- a/README.md
+++ b/README.md
@@ -223,6 +223,9 @@ Run it:
   - Debug build is slower, but gives much more useful output in a debugger.
 - If you build a bare server you don't need to have the Irrlicht or IrrlichtMt library installed.
   - In that case use `-DIRRLICHT_INCLUDE_DIR=/some/where/irrlicht/include`.
+- IrrlichtMt can also be installed somewhere that is not a standard install path.
+  - In that case use `-DCMAKE_PREFIX_PATH=/path/to/install_prefix`
+  - The path must be set so that `$(CMAKE_PREFIX_PATH)/lib/cmake/IrrlichtMt` exists.
 
 ### CMake options