libvdwxc: build system and tests
This merge request should resolve any remaining issues with the libvdwxc interface so the feature is no longer considered experimental.
Adds m4 file and other infrastructure for linking against libvdwxc through
Adds two unit tests of libvdwxc: One non-periodic (H2O) and one 3D periodic (primitive Be HCP cell, to verify non-trivial non-orthogonal cells).
Closes: #122 (closed)