It's possible to use C preprocessor statements in a spectre netlist (type man cpp for more details). Things like:
If you do this, then you need to pass -E on the spectre command line to tell spectre that it needs to run through cpp first. You can also do things like "spectre -DSOMETHING" to define a macro for cpp.
For more information, look at "spectre -h" or read the spectre user guide and reference guide.