ENH: Support install-tree use of RTK when built externally

[axel-grc]

After some tests, it seems possible to install standalone modules, so I removed the error message.

Close #957

[SimonRit]

That doesn't work for me. If I compile and install ITK without RTK, then compile and install RTK, both with the same CMAKE_INSTALL_PREFIX, then try to compile the FirstReconstruction example againt this install directory, I get:

  Target "FirstReconstruction" links to:

    ITK::RTKModule

  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.



CMake Error at CMakeLists.txt:34 (target_link_libraries):
  Target "FirstCudaReconstruction" links to:

    ITK::RTKModule

  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.

On Ubuntu with ITK main and CMake 3.29. Don't you have the same issue? This is precisely what motivated this check. BTW, I had to implement some fixes for lpsolve with other configurations, see #976.

[axel-grc]

I don't know why it worked on my side, I maybe not tested it on a fresh environment, but you are right.
but now it should work.
The install(EXPORT ...) in ITKModuleExternal.cmake doesn't generate install code for externally-built modules. The fix adds an explicit install(EXPORT ...) to Modules/Targets/