make build of mct, gptl, pio optional

[jedwards4b]

I am planning to introduce an enhancement such that if a valid library version of these libraries is available on a system then it will be used and the cesm provided source would not be built.

[ekluzek]

@jedwards4b just some questions. Is it going to check what version of each of these is installed? And if not could there be version incompatibility issues where you couldn't use the installed version? That could be either because a feature isn't available in one version, or because of a difference in answers. Also sometimes small differences in the build can be problematic, so it'll need to select a version compatible with the CESM build. I wonder if this should be an optional thing like how the ESMF build is used in CESM? Just throwing those things out there...

[jedwards4b]

The idea is to define this in config_machines.xml by, for example, loading a module so that we
would control which library versions are available to a given cesm version.

[jedwards4b]

PIO is done: if a module exists which defines the following environment variables then the pio build will be skipped and the installed module will be used:

PIO_LIBDIR
PIO_VERSION_MAJOR
PIO_TYPENAME_VALID_VALUES
PIO_INCDIR

PIO_VERSION_MAJOR should be 1 or 2, the installed library will only be used if this number matches PIO_VERSION in the case.
PIO_TYPENAME_VALID_VALUES is a list of the backend libraries supported by this build - allowed values are netcdf,pnetcdf,netcdf4p,netcdf4c

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[rljacob]

Would still like this for MCT and gptl

[jedwards4b]

PIO build is already optional.