Exodus: skip the -G "Unix Makefiles" part (#5906)

* Exodus: skip the -G "Unix Makefiles" part

The problem is that spack passes -G "Unix Makefiles" into cmake, which normally
works. But in the Exodus package, it is being passed into a bash wrapper
script. In there, the $@ then loses the information about "Unix Makefiles"
being just one argument, and in effect passes -G Unix Makefiles into the cmake
(without quotes), and so cmake only sees -G Unix, and then fails. This is a
known problem with bash with no simple solutions. As a workaround, this patch
skips the first two arguments, i.e., -G and "Unix Makefiles". This makes it
work.

Fixes #5895.

* Port exodusii to cmake

The cmake options were taken from the cmake-exodus bash script and ported to
spack directly.

* Use variant forwarding to forward the 'mpi'

Now instead of

    spack install exodusii~mpi^netcdf~mpi^hdf5~mpi

one can just use

    spack install exodusii~mpi

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