conflict in Nifti library when building with installed ITK

[KrisThielemans]

Building with system ITK (installed via conda, currently ITK 5.3), NiftyReg and ITK both use the nifti library and are not careful about conflicts, leading to

In file included from /home/kris/devel/install/include/nifti1_io.h:18,
                 from /home/kris/devel/install/include/_reg_maths.h:22,
                 from /home/kris/devel/install/include/_reg_tools.h:20,
                 from /home/kris/devel/buildConda/sources/SIRF/src/Registration/cReg/include/sirf/Reg/NiftiImageData.h:40,
                 from /home/kris/devel/buildConda/sources/SIRF/src/Registration/cReg/include/sirf/Reg/NiftiImageData3D.h:32,
                 from /home/kris/devel/buildConda/sources/SIRF/src/Synergistic/cSyn/utilities.cpp:34:
/home/kris/devel/install/include/nifti1.h:144:11: error: redefinition of 'struct nifti_1_header'
  144 |    struct nifti_1_header
      |           ^~~~~~~~~~~~~~
In file included from /home/kris/miniconda3/envs/cilsirfbuild/include/ITK-5.3/nifti1_io.h:18,
                 from /home/kris/devel/buildConda/sources/SIRF/src/Registration/cReg/include/sirf/Reg/NiftiImageData.h:32:
/home/kris/miniconda3/envs/cilsirfbuild/include/ITK-5.3/nifti1.h:148:8: note: previous definition of 'struct nifti_1_header'
  148 | struct nifti_1_header { /* NIFTI-1 usage         */  /* ANALYZE 7.5 field(s) */
      |        ^~~~~~~~~~~~~~

I'm not sure though why ITK is in the include path here.