Building gcc on powerpc-apple-darwin9, I got the following error:

...
/opt/gcc/darwin_buildw/./prev-gcc/xgcc -B/opt/gcc/darwin_buildw/./prev-gcc/ -B/opt/gcc/gcc4.4w/powerpc-apple-darwin9/bin/ -c  -g -O2 -mdynam
ic-no-pic -DIN_GCC   -W -Wall -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition -Wmissing-format-attribute -pe
dantic -Wno-long-long -Wno-variadic-macros                                   -Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -I
. -I. -I../../gcc-4.4-work/gcc -I../../gcc-4.4-work/gcc/. -I../../gcc-4.4-work/gcc/../include -I../../gcc-4.4-work/gcc/../libcpp/include -I/
sw/include  -I../../gcc-4.4-work/gcc/../libdecnumber -I../../gcc-4.4-work/gcc/../libdecnumber/dpd -I../libdecnumber  ../../gcc-4.4-work/gcc/
calls.c -o calls.o
cc1: warnings being treated as errors
../../gcc-4.4-work/gcc/calls.c: In function 'compute_argument_block_size':
../../gcc-4.4-work/gcc/calls.c:1192: error: unused parameter 'fndecl'
../../gcc-4.4-work/gcc/calls.c: In function 'emit_library_call_value_1':
../../gcc-4.4-work/gcc/calls.c:3284: error: unused variable 'fndecl'
make[3]: *** [calls.o] Error 1
make[2]: *** [all-stage2-gcc] Error 2
make[1]: *** [stage2-bubble] Error 2
make: *** [all] Error 2

And bootstraping on i686-apple-darwin9 fails with:

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/opt/gcc/i686-darwin/./prev-gcc/xgcc -B/opt/gcc/i686-darwin/./prev-gcc/ -B/opt/gcc/gcc4.4w/i686-apple-darwin9/bin/ -c  -g -O2 -fomit-frame-pointer -DIN_GCC   -W -Wall -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition -Wmissing-format-attribute -pedantic -Wno-long-long -Wno-variadic-macros                                      -Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -I. -I. -I../../gcc-4.4-work/gcc -I../../gcc-4.4-work/gcc/. -I../../gcc-4.4-work/gcc/../include -I./../intl -I../../gcc-4.4-work/gcc/../libcpp/include -I/sw/include  -I../../gcc-4.4-work/gcc/../libdecnumber -I../../gcc-4.4-work/gcc/../libdecnumber/dpd -I../libdecnumber  ../../gcc-4.4-work/gcc/c-common.c -o c-common.o
In file included from ../../gcc-4.4-work/gcc/c-common.c:50:
../../gcc-4.4-work/gcc/target-def.h:548:1: error: "TARGET_RETURN_IN_MEMORY" redefined
In file included from ./tm.h:5,
                 from ../../gcc-4.4-work/gcc/c-common.c:24:
../../gcc-4.4-work/gcc/config/i386/i386.h:1281:1: error: this is the location of the previous definition
...