HPCC-11798 Allow MAXLENGTH on BUILD (forward compatibility)

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6156 from ghalliday/issue11798

HPCC-11798 Allow MAXLENGTH on BUILD (forward compatibility)

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>