Problem with Macros...

Hi All,
I am having some problem with Macros. I will try to explain it with
a example.

There is a macro “errno” in stddef.h which is a standard include file
provided with microsoft. The problem comes when I want to define my own
version of “errno”. For this I defined “#undef errno” but until this #undef
gets executed, the old version of “errno” in stddef.h is compiled. Hence I
have two version of “errno” in my project. What project setting I have to
modify so that MY definition of “errno” is used for all files in the project
?
Thanks in Advance
KD