[Qt-interest] How do I tell with qmake whether system is 64 or 32-bit?

[Nikos Chantziaras]

In a qmake-based project, I need to include different libraries in the 
build if the current system is 64-bit.  Something like this:

   64bit {
       LIBS += fmodex64
   }
   32bit {
       LIBS += fmodex32
   }

How do I do that?