remove external dependencies that are sometimes problematic
libxml2 is currently required for handling reference data for tests, which means users and developers need a version available that can pass our linking test and will work when compiling our actual code. That isn't straightforward - it works best to disable support for zlib and python when configuring libxml2, but this is all a hassle. Instead we could use one of several freely available lightweight XML-handling libraries and avoid such problems. We can leave the infrastructure for libxml2 handling in place while patches in Gerrit might still want it.
zlib is an optional dependency for TNG, but has a permissive license and builds warning-free (or nearly) with recent gcc and clang, so we could just bundle zlib with TNG, and thus GROMACS. Magnus Lundborg this this may be reasonable and will try to look into it.
Update bundled TNG
This commit bundles 6e41191016 from the TNG repo.
Required TNG 1.7.10 in future, which is the first version to include
support for the build system features used here.
Update the build system to use imported targets and interface properties
that TNG now provides. Add support for building TNG that also bundles
zlib, which reduces the dependency footprint of GROMACS, which is
important for portability and ease of installation.