Thorns HDF5 and libxml2 require the zlib library. This library is currently configured ad-hoc. I just committed a new thorn ExternalLibraries/zlib that can either configure the zlib library, or that can build it if the system does not provide a usable one.
I suggest to make thorns HDF5 and libxml2 require the zlib thorn. (libxml2 is among the thorns I suggested earlier to include in the standard Einstein Toolkit thorn list.)
People will then have to update their thorn list to include the zlib thorn, and will have to configure their zlib location explicitly via this thorn. If this thorn is not included, then thorn HDF5 and libxml2 will fail to build. By default (if it is not explicitly configured), the zlib thorn will look for the zlib library in the "usual" places.
-erik