Fixes #2499: Allow ExtData2G to be built static

[mathomp4]

Description

Fixes the CMake in ExtData2G to allow it to be built as STATIC

Related Issue

Closes #2499

Motivation and Context

Our UFS friends build MAPL as STATIC and this fixes a bug with that.

How Has This Been Tested?

Built the model with -DBUILD_SHARED_MAPL=NO and ran GEOSgcm.x. Zero-diff.

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