[CMake] Use an object library for STIR_REGISTRIES

[KrisThielemans]

Using an object library means that the registries get compiled only once, therefore speeding up the build process quite a lot.

It's also easier to use when building a project that uses STIR as we just need to depend on STIR_LIBRARIES. However, we do set the STIR_REGISTRIES variable empty at export, such that external libraries can still use

  add_executable(bla ${STIR_REGISTRIES})

Fixes #83

[KrisThielemans]

This worked and resulted in great speed-up. Sadly, it doesn't work for SIRF as its executables now do not have the stir registries at all. The reason is that target_link_libraries doesn't use transitive behaviour for OBJECT libraries. Extensive discussion at https://gitlab.kitware.com/cmake/cmake/-/issues/18090

(I also have some linking errors with the current STIR commit related to HDF5, but not sure if these are related)

[KrisThielemans]

The previous commit re-enables the requirement to use ${STIR_REGISTRIES} in external projects to avoid problems with the transitive behaviour. STIR_REGISTRIES is now set (in STIRConfig.cmake) to $<TARGET_OBJECTS:stir_registries>` which works fine on my Linux VM. Fingers crossed.

[KrisThielemans]

Currently, the exported STIRConfig.cmake will either work for CMAKE < 3.12 or > 3.12, but not both.

[KrisThielemans]

This is finished (aside from release notes). @danieldeidda @markus-jehl @robbietuk @NikEfth could you try this one out please? Especially if you have something that uses find_package(STIR).

[markus-jehl]

Just tested it and it works fine, as far as I can tell.