Unity: Proper handling of object libraries

Fixes: #20051

Tests/RunCMake/UnityBuild/RunCMakeTest.cmake

@@ -21,3 +21,4 @@ function(run_test name)
 endfunction()
 
 run_test(unitybuild_runtest)
+run_test(unitybuild_object_library)

Tests/RunCMake/UnityBuild/unitybuild_object_library.cmake

@@ -0,0 +1,13 @@
+project(unitybuild_object_library C)
+
+set(CMAKE_UNITY_BUILD ON) # This tests that the variable works in addition to the property
+
+add_library(lib OBJECT func.c)
+
+add_library(other-lib STATIC func.c)
+
+add_executable(main main.c)
+target_link_libraries(main PRIVATE lib)
+
+enable_testing()
+add_test(NAME main COMMAND main)