Sync the cmake arguments to search for DB2 with the rest of the qsql
find modules and provide a DB2_ROOT env or cmake var.
Also allow the old variables as a fallback.
Pick-to: 6.10
Change-Id: I587e519b5cf3513e9580f64f0fb9b46bf789da5c
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
4832f61212