Synthesis flags as variables

Synthesis page is just toggles over synth_xilinx, but shrike-gen hardcodes one fixed combination in the Makefile. Lifting those into per-project variables (FLATTEN/NODSP/NOBRAM/ABC9/AUTONAME/WIDEMUX) with GUI-matching defaults, makes builds directly comparable to the GUI.

Ref: https://github.com/annoyedmilk/LogicCard-VHDL/issues/1