Add NetCDF C requirement to CMake

[mathomp4]

Description

As found by @climbfuji in JCSDA/spack#111 MAPL actually does have NetCDF C code. For some reason, their spack builds found this but mine didn't.

Still, it is plainly obvious we use NetCDF C so we must say so in our CMake!

Related Issue

Motivation and Context

Will let our NOAA-EMC friends not need to carry around a patch for future MAPL.

How Has This Been Tested?

This should only affect non-Baselibs builds so we should be good. I suppose we'll find out if this worked when @climbfuji updates MAPL!

I might also ping @weiyuan-jiang to try on Orion.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Trivial change (affects only documentation or cleanup)

Checklist:

• I have tested this change with a run of GEOSgcm (if non-trivial)
• I have added one of the required labels (0 diff, 0 diff trivial, 0 diff structural, non 0-diff)
• I have updated the CHANGELOG.md accordingly following the style of Keep a Changelog