[R-SIG-Mac] -F option in Makeconf

[Cyrus Harmon]

if one installs the R framework somewhere other than /Library or / 
System/Library, Makeconf needs a -F<path to the directory in which  
R.framework is installed> argument before the -framework R argument  
on the LIBR line. I'm not sure the best way to do that. Looks like  
adding the equivalent of -F${prefix} to the WANT_R_FRAMEWORK_TRUE  
line that sets LIBR would work, but there's probably a better way to  
set the framework path at configure time.

Should this go to r-devel or here?

Thanks,

Cyrus