IMSL and MKL selection from compiler directives

I'm using Intel Visual Fortran v.11.1.065 under Windows 7 64 bit. 
What I want to make is related with compiler directives, especially for math library.
Here's my idea :

1. There exist two kinds of debug configuration, for IMSL and MKL. 
Thus command line arguments contains library-specific environment variables.
2. In code, compiler directive determine which math library is selected.
Then, library-specific functions can also be selected in code.

However, I don't know how to set the compiler directives for determining libraries.

Thanks, 
HK