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.

.gitlab-ci.yml

@@ -9,7 +9,7 @@ stages:
   - deploy
 
 variables:
-  IRRLICHT_TAG: "1.9.0mt1"
+  IRRLICHT_TAG: "1.9.0mt2"
   MINETEST_GAME_REPO: "https://github.com/minetest/minetest_game.git"
   CONTAINER_IMAGE: registry.gitlab.com/$CI_PROJECT_PATH
 
@@ -19,14 +19,10 @@ variables:
    - apt-get update
    - apt-get -y install build-essential git cmake libpng-dev libjpeg-dev libxxf86vm-dev libgl1-mesa-dev libsqlite3-dev libleveldb-dev libogg-dev libvorbis-dev libopenal-dev libcurl4-gnutls-dev libfreetype6-dev zlib1g-dev libgmp-dev libjsoncpp-dev
   script:
-    - git clone https://github.com/minetest/irrlicht -b $IRRLICHT_TAG
-    - cd irrlicht
-    - cmake . -DBUILD_SHARED_LIBS=OFF
-    - make -j2
-    - cd ..
+    - git clone https://github.com/minetest/irrlicht -b $IRRLICHT_TAG lib/irrlichtmt
     - mkdir cmakebuild
     - cd cmakebuild
-    - cmake -DIRRLICHT_LIBRARY=$PWD/../irrlicht/lib/Linux/libIrrlichtMt.a -DIRRLICHT_INCLUDE_DIR=$PWD/../irrlicht/include -DCMAKE_INSTALL_PREFIX=../artifact/minetest/usr/ -DCMAKE_BUILD_TYPE=Release -DRUN_IN_PLACE=FALSE -DENABLE_GETTEXT=TRUE -DBUILD_SERVER=TRUE ..
+    - cmake -DCMAKE_INSTALL_PREFIX=../artifact/minetest/usr/ -DCMAKE_BUILD_TYPE=Release -DRUN_IN_PLACE=FALSE -DENABLE_GETTEXT=TRUE -DBUILD_SERVER=TRUE ..
     - make -j2
     - make install
   artifacts: