cmake tweak: set the HDF5_ROOT environment variable if necessary
cmake tweak: set the HDF5_ROOT environment variable if necessary

It turns out that FindHDF5 searches the HDF5_ROOT environment variable
rather than the equivalent cmake variable. So if the user specifies the
cmake variable, then we simply set the same environment variable before
calling find_package.

ulrikpedersen/swmr-testapp,ulrikpedersen/swmr-testapp,ulrikpedersen/swmr-testapp