Fix for the fact we find_package(AAX) all the time.

Since we probe for AeonWave first, we set AAX_LIBRARY to NOT_FOUND,
which then upsets CMake if we try to use it. So add some temporary
ugly logic, until I have time to define a proper ALIAS target for the
audio backend.

simgear/CMakeLists.txt

@@ -163,10 +163,19 @@ if(NOT SIMGEAR_HEADLESS)
 
     target_link_libraries(SimGearScene PRIVATE
         ZLIB::ZLIB
-        ${OPENAL_LIBRARY} ${AAX_LIBRARY}
         ${OPENGL_LIBRARY}
         ${JPEG_LIBRARY})
 
+    # ToDo: define an ALIAS target for the sound backend, to get
+    # rid of this logic here
+    if (ENABLE_SOUND)
+        if (USE_AEONWAVE)
+            target_link_libraries(SimGearScene PRIVATE ${AAX_LIBRARY})
+        else()
+            target_link_libraries(SimGearScene PRIVATE ${OPENAL_LIBRARY} )
+        endif()
+    endif()
+
     if(ENABLE_GDAL)
         # FIXME: should be RIVATE, but we leak GDAL headers
         # in our own headers :(