Cannot find /lib/libc.so.6

You didn't indicate what gcc version you are using, but if it is a recent enough one (4.0.0 and above me thinks) you should try adding the --sysroot flag to g++/ld. Point it to $SYSROOT as defined in your Makefile. For example:

--sysroot=$(SYSROOT)

Assuming recent enough gcc version, it will work.

I just went through the same issue; adding --sysroot=/rootfs/prefix helped me get closer to the real issue. I got it fixed by installing package libstdc++-dev in target.