invalid dependency information in cache when converting static library to interface library

when converting a static library foo to an interface library, the cache entry for the lib dependencies is stale:

CMake Error: Target foo has dependency information when it shouldn't.
Your cache is probably stale. Please remove the entry
  foo_LIB_DEPENDS

while it is easy to remove the cache entry or restart with a new buildfolder, it is a rather obscure error that users of the library can run into.