this is the output:

mkdir alphaev68-dec-osf4.0g/libffi
Configuring in alphaev68-dec-osf4.0g/libffi
[...]
checking whether /usr/users/syajnl/soft/gcc-build/gcc/xgcc
-B/usr/users/syajnl/soft/gcc-build/gcc/ -B/usr/local/alphaev68-dec-osf4.0g/bin/
-B/usr/local/alphaev68-dec-osf4.0g/lib/ -isystem
/usr/local/alphaev68-dec-osf4.0g/include -isystem
/usr/local/alphaev68-dec-osf4.0g/sys-include accepts -g... (cached) yes
/usr/users/syajnl/soft/gcc-3.4.0/libffi/configure: test: argument expected
gmake[1]: *** [configure-target-libffi] Error 1
gmake[1]: Leaving directory `/usr/users/syajnl/soft/gcc-build'
gmake: *** [bootstrap] Error 2

after adding "set -x" to the relevant configure:
[...]
+ echo ${ac_cv_prog_cc_g+set}
+ eval test "${ac_cv_prog_cc_g+set}" = set
+ test set = set
+ echo (cached) \c
(cached) + echo yes
yes
+ test set = set
CFLAGS=-O2 -g -O2 -mieee
+ test != yes

Hope this helps.