ISMRMRD target_include_dir only if no ISMRMRD::ISMRMRD target

[rijobro]

Fixes #659 (maybe)

[KrisThielemans]

feels a bit weird actually to specify a link target to tell CMake what to do with include files. probably it'd be better to have

target_include_directories(cgadgetron PUBLIC ISMRMRD::ISMRMRD)

but the CMake doc doesn't make it clear if this can be done.

Seems I'm pushing my CMake knowledge with this one...

[rijobro]

My understanding was that an imported library would be created during the *Targets.cmake, which would itself use the target_include_directories. Then you could use target_link_libraries in your project and it would already know which headers are needed.

E.g. in ISMRMRDTargets.cmake, I have:

add_library(ISMRMRD::ISMRMRD SHARED IMPORTED)

set_target_properties(ISMRMRD::ISMRMRD PROPERTIES
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include/ismrmrd"
  INTERFACE_LINK_LIBRARIES "/usr/local/Cellar/hdf5/1.12.0/lib/libhdf5.dylib;/usr/local/opt/szip/lib/libsz.dylib;/usr/lib/libz.dylib;/usr/lib/libdl.dylib;/usr/lib/libm.dylib"
)

[KrisThielemans]

superseded by #840