cmake: make OpenAL an imported target

cmake/modules/FindOpenAL.cmake

@@ -0,0 +1,13 @@
+# Override CMake's FindBullet module:
+# create an IMPORTED TARGET
+
+include("${CMAKE_ROOT}/Modules/FindOpenAL.cmake")
+
+if(OPENAL_FOUND)
+  add_library(OpenAL::OpenAL UNKNOWN IMPORTED)
+  set_target_properties(OpenAL::OpenAL PROPERTIES
+    IMPORTED_LINK_INTERFACE_LANGUAGES "C;CXX"
+    IMPORTED_LOCATION "${OPENAL_LIBRARY}"
+    INTERFACE_INCLUDE_DIRECTORIES "${OPENAL_INCLUDE_DIR}"
+    )
+endif()

rwengine/CMakeLists.txt

@@ -151,14 +151,7 @@ target_link_libraries(rwengine
         bullet::bullet
         ffmpeg::ffmpeg
         glm::glm
-    PRIVATE
-        ${OPENAL_LIBRARY}
-    )
-
-target_include_directories(rwengine
-    SYSTEM
-    PUBLIC
-        ${OPENAL_INCLUDE_DIR}
+        OpenAL::OpenAL
     )
 
 target_include_directories(rwengine

rwgame/CMakeLists.txt

@@ -49,7 +49,6 @@ target_include_directories(rwgame
     SYSTEM
     PRIVATE
         ${Boost_INCLUDE_DIRS}
-        ${OPENAL_INCLUDE_DIR}
     )
 
 target_include_directories(rwgame