Fix missing Lua::Lua target when using system Lua

[syzygial]

Enabling tests/examples when using an installed version of Lua results in

CMake Error at examples/CMakeLists.txt:26 (target_link_libraries):
  Target "..." links to:

    Lua::Lua

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.