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.

util/buildbot/buildwin32.sh

@@ -30,7 +30,7 @@ if [ -z "$toolchain_file" ]; then
 fi
 echo "Using $toolchain_file"
 
-irrlicht_version=1.9.0mt1
+irrlicht_version=1.9.0mt2
 ogg_version=1.3.4
 vorbis_version=1.3.7
 curl_version=7.76.1
@@ -97,7 +97,7 @@ cd $builddir
 mkdir build
 cd build
 
-irr_dlls=$(echo $libdir/irrlicht/bin/*.dll | tr ' ' ';')
+irr_dlls=$(echo $libdir/irrlicht/lib/*.dll | tr ' ' ';')
 vorbis_dlls=$(echo $libdir/libvorbis/bin/libvorbis{,file}-*.dll | tr ' ' ';')
 gettext_dlls=$(echo $libdir/gettext/bin/lib{intl,iconv}-*.dll | tr ' ' ';')
 
@@ -113,9 +113,7 @@ cmake -S $sourcedir -B . \
 	-DENABLE_FREETYPE=1 \
 	-DENABLE_LEVELDB=1 \
 	\
-	-DIRRLICHT_INCLUDE_DIR=$libdir/irrlicht/include/irrlichtmt \
-	-DIRRLICHT_LIBRARY=$libdir/irrlicht/lib/libIrrlichtMt.dll.a \
-	-DIRRLICHT_DLL="$irr_dlls" \
+	-DCMAKE_PREFIX_PATH=$libdir/irrlicht \
 	\
 	-DZLIB_INCLUDE_DIR=$libdir/zlib/include \
 	-DZLIB_LIBRARIES=$libdir/zlib/lib/libz.dll.a \