[Haskell-cafe] How to define CPP macro definition with stack?

[PY]

Hello all,

I'm using conditional compilation:

#ifdef BLAH
   blahBlah
   #endif

And I want to define BLAH, but not in *.hs file. For example, with 
`stack` option or as environment variable... How can I do it? For 
example, `stack -DBLAH=1 build` or something else.
I tried

   stack build --flag='*:BLAH'

- no effect.

Also I tried

   stack build --flag=my-library:BLAH

and get error that my-library package does not define flag BLAH. So, 
it's total secret for me how to make such thing (like we can do with 
gcc, make, etc)  :-)

===

Best regards, Paul