Configuring with 

./configure --with-gcc="gcc -m64 -L/usr/sfw/lib/64 -R/usr/sfw/lib/64" 

where 64bit libreadline is found in /usr/sfw/lib/64, configure script fails to realize that readline lib is present.

I was able to fix the problem by editing config script manually as follows:

$ diff configure~ configure
20744c20744
< if test "$ac_cv_have_readline_readline" = no
---
> if test "$ac_cv_lib_readline_readline" = no
20747c20747
< echo "$as_me:$LINENO: checking for readline in -ltermcap" >&5
---
> echo "$as_me:$LINENO: checking for readline in -lreadline -ltermcap" >&5
20753c20753
< LIBS="-ltermcap  $LIBS"
---
> LIBS="-lreadline -ltermcap  $LIBS"


A proper fix would be to change configure.in, but I don't know enough about autoconf to submit a proper patch.