Vivado HLS build broken

[thesps]

PR #53 probably broke synthesis for Vivado HLS (while it works with more recent Vitis HLS).

In the HLS build log:

ERROR: [HLS 200-101] 'open_solution': Unknown option '-flow_target'.

This will need some care to find a solution that works across all versions, but could be handled either in the Python HLS backend writer or the TCL script itself.