GLSL - pre-processor?

Is there any any support for pre-processor in GLSL, like HLSL? I am specifically talking about “ifdefs”, “ifndefs” etc. ?
I cannot figure out a way since none of the functions that i have studied take any sort of input specific to any pre-processing step. Any help in this regard will be highly appreciated.

Off the main OpenGL page is a link to the GLSL spec:

Goto page 9