Make um7 path definitions more consistent with other ACCESS-NRI packages

[penguian]

The current path definitions for um7 use:

• PATH to locate fcm.
• env.prepend_path("CPATH", ...) to define paths to headers and Fortran modules.
• env.prepend_path("LIBRARY_PATH", ...) to define paths to libraries at build time.
• env.prepend_path("LD_LIBRARY_PATH", ...) to define the path to netcdf-fortran at runtime.

Similarly to cice4 and other ACCESS-NRI Spack package definitions:

• Using depends_on("fcm", ...) would be preferable, since it allows for versioning.
• Paths to headers and Fortran modules would be better defined similar to the following from cice4.

  istr = " ".join([
              join_path((spec["oasis3-mct"].headers).cpp_flags, "psmile.MPI1"),
              join_path((spec["oasis3-mct"].headers).cpp_flags, "mct")])
  ideps = ["netcdf-fortran",...]
  incs = " ".join([istr] + [(spec[d].headers).cpp_flags for d in ideps])

• Library paths should be specified via RPATH, similar to the following from cice4.

  def get_linker_args(self, spec, name):
      return " ".join(
                  [(spec[name].libs).ld_flags,
                  "-Wl,-rpath=" + join_path(spec[name].prefix, "lib")]
                 )
  ...
      lstr = ""
      ldeps = ["oasis3-mct", "netcdf-fortran"]
      libs = " ".join([lstr] + [self.get_linker_args(spec, d) for d in ldeps])

See also #113 for another um7-related issue.