determining 32 v. 64 bit compilation

[Wesley Smith]

What's the appropriate way to determine 32 v. 64 bit compilation?
I've looked into using:

AC_CANONICAL_BUILD

with the $build_cpu $build_vendor $build_os variables, but it doesn't
give the right info (in my case i686 pc linux-gnu).  I'm essentially
looking for something in autoconf land equivalent to:

#if defined(__LP64__) || defined(_LP64)
        #define OSBIT 64;
    #else
        #define OSBIT 32;
    #endif


thanks in advance!
wes