Specifying per-file command-line compiler option

I am creating a library using F#, and would like to generate .fsi signature files for each F# source file. To do this, I think I just need to add this command-line option for each file:

--sig:<filename>.fsi

How can I do that? In the Project Options > Compiler > Command Line Parameters field, I can add parameters, but don't know how to specify the current source file's filename. Is there some sort of environment variable available?

Alternatively, is there some way to specify command-line arguments for each source file?