Victor, please do not use magic constants like that in C.  The symbolic values are available in include files:

#include <CoreServices/CoreServices.h>
SInt32 major = 0;
SInt32 minor = 0;   
Gestalt(gestaltSystemVersionMajor, &major);
Gestalt(gestaltSystemVersionMinor, &minor);
if ((major == 10 && minor >= 7) || major >= 11) { ... }

(See, for instance, http://www.cocoadev.com/index.pl?DeterminingOSVersion and http://stackoverflow.com/questions/2115373/os-version-checking-in-cocoa. The code in platform and _gestalt.c could stand to be updated at some point.)

But, again, mmap should *not* be changed until 10.7 has been released and the Apple fix is verified and only if it makes sense to add the additional complexity.