[boost] [build] adding a rquirement for Cygwin only?

Is it possible to add, for example, <threadapi>pthread to a project's requirements, but only for Cygwin? This works: <toolset>gcc,<target-os>windows:<threadapi>pthread but it also applies to MinGW. This only applies to MinGW: toolset-gcc:flavormingw:<threadapi>pthread but Cygwin has no <flavor>. This: <toolset>gcc,<target-os>windows:<threadapi>pthread toolset-gcc:flavormingw:-<threadapi>pthread doesn't work. There are references to <target-os>cygwin in gcc.jam, but <target-os> is actually 'windows'.