Updated hdf5/CMakeLists.txt so it tries to use HDF5_ROOT_DIR_HINT

This environment variable (recommended in FindHDF5.cmake) can be used to automatically find HDF5.

The TO_CMAKE_PATH will ensure that it works on any platform. Tested on Windows using an install path containing spaces.

Unfortunately, the Windows HDF5 installer does not create this environment variable, but I'll create a JIRA entry to rectify that.