[Rd] Can configure.ac detect 64 bit R?

[jhallman at frb.gov]

My fame package has to link to the libchli.so that comes with FAME.
However, FAME is now supplying both 32 and 64 bit versions of the
library. The 32-bit version is $FAME/hli/libchli.so while the 64-bit
version is $FAME/hli/64/libchli.so.  To set the right flags, it seems
that I need to know, from within configure.ac, whether the R
installation is 32 bit or 64 bit.  Is there a way to detect this?

Jeff