The important part of the log is:
/bin/sh ./libtool --tag=CC --mode=compile
/Users/regress/tbox/native/build/./gcc/xgcc
-B/Users/regress/tbox/native/build/./gcc/
-B/Users/regress/tbox/objs/powerpc-apple-darwin9.8.0/bin/
-B/Users/regress/tbox/objs/powerpc-apple-darwin9.8.0/lib/ -isystem
/Users/regress/tbox/objs/powerpc-apple-darwin9.8.0/include -isystem
/Users/regress/tbox/objs/powerpc-apple-darwin9.8.0/sys-include
-DHAVE_CONFIG_H -I. -I/Users/regress/tbox/svn-gcc/boehm-gc -I./include
-I/Users/regress/tbox/svn-gcc/boehm-gc/include -fexceptions
-Iinclude -I././targ-include -I.//libc/include -g -O2 -mtune=generic
-c -o allchblk.lo /Users/regress/tbox/svn-gcc/boehm-gc/allchblk.c
libtool: compile: /Users/regress/tbox/native/build/./gcc/xgcc
-B/Users/regress/tbox/native/build/./gcc/
-B/Users/regress/tbox/objs/powerpc-apple-darwin9.8.0/bin/
-B/Users/regress/tbox/objs/powerpc-apple-darwin9.8.0/lib/ -isystem
/Users/regress/tbox/objs/powerpc-apple-darwin9.8.0/include -isystem
/Users/regress/tbox/objs/powerpc-apple-darwin9.8.0/sys-include
-DHAVE_CONFIG_H -I. -I/Users/regress/tbox/svn-gcc/boehm-gc -I./include
-I/Users/regress/tbox/svn-gcc/boehm-gc/include -fexceptions -Iinclude
-I././targ-include -I.//libc/include -g -O2 -mtune=generic -c
/Users/regress/tbox/svn-gcc/boehm-gc/allchblk.c -fno-common -DPIC -o
.libs/allchblk.o
/Users/regress/tbox/svn-gcc/boehm-gc/allchblk.c:1:0: error: bad value
(generic) for -mtune= switch