cygwin with --disable-threadsafe

[Robert Collins]

Following up on my questions about @PTH_ALLOW@ I tried build cygwin with
--disable-threadsafe. It didn't build - pinfo and signal.h appeared to
be the main culprits.

Have I broken something with my changes (I haven't removed the stub
functions that are under discussion yet), or is it currently broken? And
yes I plan to test it, but I may have something wrong here so an outside
reference point is useful.

Rob