mcs.rsp File Not Working

I'm trying to define preprocessor directives using the mcs.rsp file. I'm on windows, using c#. I followe the directions delineated here (at the bottom of the page). The file looks exactly like this:

 -define:DIRECTIVE

A simple test of this nature:

 #if DIRECTIVE
      //do something
 #else
      //do something else
 #endif

indicates that the directive is not seen by the compiler.

In case anyone is curious, the reason I would like to create the directive is to determine, precompilation, whether we are playing from the editor, or from the command line. If there is another way to do this, that would also work for me.

As a side note, sorry about the line spacing in this question. My new lines are not being recognized for some reason.