Determining OS type and perl version

[Sisyphus]

Hi,
Where do I find the definitions (assuming such exist) that I can use to
determine from within an XSub:

1) The operating system in use (specifically, whether it's Cygwin);
2) The version of perl (specifically, whether it's >= 5.6.1);

In the past I've used the values of $] and $^O to DEFINE my own symbols in
the Makefile.PL - which works fine, but is a little silly if the info the
XSub needs is already available.

Cheers,
Rob