MDEV-24883 follow up: unset variables in CMake

cmake/uring.cmake

@@ -12,8 +12,13 @@ MACRO(CHECK_URING)
         IF(WITH_URING STREQUAL "yes")
           MESSAGE(FATAL_ERROR "Requested WITH_URING=yes but liburing was not found")
         ENDIF()
+        UNSET(LIBURING CACHE)
+        UNSET(HAVE_LIBURING_H CACHE)
       ENDIF()
-    ELSEIF(NOT WITH_URING STREQUAL "no")
+    ELSEIF(WITH_URING STREQUAL "no")
+      UNSET(LIBURING CACHE)
+      UNSET(HAVE_LIBURING_H CACHE)
+    ELSE()
       MESSAGE(FATAL_ERROR "Invalid value for WITH_URING. Must be 'yes', 'no', or 'auto'.")
     ENDIF()
   ENDIF()