NIOS2 rebuild toolchain

Has anybody an idea why rebuilding the toolchain fails when not compiled on Windows under Cygwin?

Always stops at the ld configure stage:

*** ld does not support target nios2-elf

*** see ld/configure.tgt for supported targets

make: *** [configure-ld] Error 1

Somehow it looses "nios2-unknown-elf"...