Depending on which path is used by FindHDF5, the HDF5_${__lang}_INCLUDE_DIRS variable is not always set. In particular, when using find_path(HDF5_${__lang}_INCLUDE_DIR ...) is used, but the (language-specific) _DIRS variable is not set. In all other cases, the _DIR variable is not set.

We should always set _DIRS. Optionally, we could set _DIR to _DIRS as I did before

foreach(__lang IN LISTS HDF5_LANGUAGE_BINDINGS)
   if ( HDF5_${__lang}_INCLUDE_DIRS )
       set(HDF5_${__lang}_INCLUDE_DIR "${HDF5_${__lang}_INCLUDE_DIRS}")
   endif()
endforeach()

but I guess @brad.king doesn't like that. Fine for me, as the _DIR variable wasn't always set since 00405af0 so users cannot rely on existence of the (deprecated) variable anyway.