COMP: Move GDCM version check to cxx file

[blowekamp]

Addresses warning about undefined GDCM_MAJOR_VERSION.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)
• Added license to new files (if any)
• Added Python wrapping to new files (if any) as described in ITK Software Guide Section 9.5
• Added ITK examples for all new major features (if any)

Refer to the ITK Software Guide for
further development details if necessary.

[blowekamp]

@seanm Can check that this addresses the issue?

[seanm]

Will certainly fix my own CI, since only ITK header warnings will appear on it. I don't recall if ITK source is -Wundef clean or not...

I vote merge :)