BUILD: thread: limit the libgcc_s workaround to glibc only

BUILD: thread: limit the libgcc_s workaround to glibc only

Previous commit 77b98220e ("BUG/MINOR: threads: work around a libgcc_s
issue with chrooting") broke the build on cygwin. I didn't even know we
supported threads on cygwin. But the point is that it's actually the
glibc-based libpthread which requires libgcc_s, so in absence of other
reports we should not apply the workaround on other libraries.

This should be backported along with the aforementioned patch.