CMake: Use openttd_lib split build from upstream

Add option to disable for cross-compiling

cmake/CheckAtomic.cmake

@@ -83,5 +83,5 @@ else()
 endif()
 
 if(HAVE_CXX_ATOMICS_WITH_LIB OR HAVE_CXX_ATOMICS64_WITH_LIB)
-  target_link_libraries(openttd atomic)
+  target_link_libraries(${OPENTTD_LIB} atomic)
 endif()

cmake/LinkPackage.cmake

@@ -8,13 +8,13 @@ function(link_package NAME)
         # which (later) cmake considers to be an error. Work around this with by stripping the incoming string.
         if(LP_TARGET AND TARGET ${LP_TARGET})
             string(STRIP "${LP_TARGET}" LP_TARGET)
-            target_link_libraries(openttd ${LP_TARGET})
+            target_link_libraries(${OPENTTD_LIB} ${LP_TARGET})
             message(STATUS "${NAME} found -- -DWITH_${UCNAME} -- ${LP_TARGET}")
         else()
             string(STRIP "${${NAME}_LIBRARY}" ${NAME}_LIBRARY)
             string(STRIP "${${NAME}_LIBRARIES}" ${NAME}_LIBRARIES)
             include_directories(${${NAME}_INCLUDE_DIRS} ${${NAME}_INCLUDE_DIR})
-            target_link_libraries(openttd ${${NAME}_LIBRARIES} ${${NAME}_LIBRARY})
+            target_link_libraries(${OPENTTD_LIB} ${${NAME}_LIBRARIES} ${${NAME}_LIBRARY})
             message(STATUS "${NAME} found -- -DWITH_${UCNAME} -- ${${NAME}_INCLUDE_DIRS} ${${NAME}_INCLUDE_DIR} -- ${${NAME}_LIBRARIES} ${${NAME}_LIBRARY}")
         endif()
     elseif(LP_ENCOURAGED)

cmake/SourceList.cmake

@@ -32,7 +32,7 @@ endfunction()
 # For example: ADD_IF SDL_FOUND AND Allegro_FOUND
 #
 function(add_files)
-    _add_files_tgt(openttd ${ARGV})
+    _add_files_tgt(${OPENTTD_LIB} ${ARGV})
 endfunction()
 
 # Add a test file to be compiled.
@@ -44,7 +44,9 @@ endfunction()
 # For example: ADD_IF SDL_FOUND AND Allegro_FOUND
 #
 function(add_test_files)
-    _add_files_tgt(openttd_test ${ARGV})
+    if(NOT OPTION_NO_SPLIT_LIB)
+        _add_files_tgt(openttd_test ${ARGV})
+    endif()
 endfunction()
 
 # This function works around an 'issue' with CMake, where