Re-Introduce "$ENV{HDF5_ROOT}" in CMAKE_PREFIX

- adding HDF5_ROOT to CMAKE_PREFIX allows to PREFER the installation in
  HDF5_ROOT over system installations
Remove LINK_DIRECTORIES for PACKAGES (HDF5)

- in FIND_PACKAGES, INCLUDE_DIRECTORIES returns absolute paths
  by default and LINK_DIRECTORIES should not be used
  http://cmake.org/cmake/help/v2.8.11/cmake.html#command:link_directories
Merge pull request #25 from ax3l/topic/prefix

Re-Introduce "$ENV{HDF5_ROOT}" in CMAKE_PREFIX