configureが違うからか？
$ /usr/local/xp/bin/g++ -v
Reading specs from /usr/local/xp/lib/gcc/i686-pc-cygwin/3.4.0/specs
Configured with: ../gcc-3.4.0/configure --enable-languages=c,c++ --enable-libgcj
--enable-threads=posix --with-system-zlib --enable-nls --without-included-gette
xt --enable-interpreter --enable-sjlj-exceptions --disable-version-specific-runt
ime-libs --enable-shared --host=i686-pc-cygwin --target=i686-pc-cygwin --prefix=
/usr/local/xp
Thread model: posix
gcc version 3.4.0