invalid BUILDMODE in cygwin 15s build

[Farid Zaripov]

  I see in
http://people.apache.org/~sebor/stdcxx/results/win_xp-2-x86-gcc-32b-3.4.
4-15s-win32-559300-log.gz.txt
that build was started with BUILDMODE=,archive,debug,narrow but should
started with
BUILDMODE=pthreads,archive,debug,narrow

Farid.