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

Subject: [boost] [build] adding a rquirement for Cygwin only?
From: Peter Dimov (lists_at_[hidden])
Date: 2015-01-19 11:34:28

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:flavor>mingw:<threadapi>pthread

but Cygwin has no <flavor>.

This:

    <toolset>gcc,<target-os>windows:<threadapi>pthread
    <toolset-gcc:flavor>mingw:-<threadapi>pthread

doesn't work.

There are references to <target-os>cygwin in gcc.jam, but <target-os> is 
actually 'windows'.