Fix issue with calling ACG multiple times

[mathomp4]

Description

This PR makes some changes to the cmake custom command that invokes the Python ACG to alleviate the no-op rebuilds of GOCART2G (see GEOS-ESM/GOCART#117)

Thanks to help from @tclune, this now should fix that issue.

Related Issue

GEOS-ESM/GOCART#117

Motivation and Context

Makes the build more efficient.

How Has This Been Tested?

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

[tclune]

Correction - the PR makes changes to the CMake ACG custom command. ACG itself is unchanged. I'll edit the PR.

[mathomp4]

Correction - the PR makes changes to the CMake ACG custom command. ACG itself is unchanged. I'll edit the PR.

Ah. Yeah. My bad :)

[mathomp4]

Testing shows this is zero-diff, so we should be safe.

[mathomp4]

Oh, and I fixed up a bit of old-style CMake I saw in here.