error: implicit declaration of function 'GetActiveProcessorCount' is invalid in C99. Make sure that you include the function prototype.

I am getting  the following error:

error: implicit declaration of function 'GetActiveProcessorCount' is invalid in C99. Make sure that you include the function prototype.

I have included windows.h

which includes winbase.h 

Winbase.h contains the prototype for the function called out in the error message above. 

If I disable 'Require function prototypes' and 'Build with C99 extensions' I get the following error:

error: Undefined symbol '_GetActiveProcessorCount' referenced in "c:\Users\Public\Documents\National Instruments\CVI\HDLC\cvibuild.HDLC_RandD\Debug\HDLC_RandD.obj".  

I am working in CVI 2013 SP2.

Why am I getting this error.