`make_target` can be used to instruct Spack to build one of the pre-defined make
targets in the MPAS makefile. Spack will guess the targret based on the value of
`spack_fc`, but the user can overwrite the target as variant. E.g.
```
spack install mpas-model make_target=pgi-nersc
```
`precision` is used to set the `PRECISION` flag in the Makefile to {single,
double}. Default is double.
Co-authored-by: Stephen Sachs <stesachs@amazon.com>
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