exported CONFIG.cmake problems

[KrisThielemans]

Using NIFTYREG_LIBRARIES fails as these are just the names of the libraries, not absolute paths, and their dependencies are not exported.
We currently work around this as in https://github.com/SyneRBI/SIRF/blob/master/src/CMakeLists.txt#L53-L71

I believe this needs

install(TARGETS somelib EXPORT NIFTYREGTargets 
   RUNTIME_DESTINATION ...)

for every library, and in cmake/CMakeLists.txt

install(EXPORT NIFTYREGTargets DESTINATION "${ConfigPackageLocation}")

Also, as opposed to doing

niftyreg/CMakeLists.txt

Lines 173 to 174 in 6db8b16

	set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")

It is now recommended to depend on OpenMP::OpenMP_C (or OpenMP::OpenMP_CXX). Linking and includes will then be automatically ok. Similar stuff for CUDA I guess.