Cygwin compile fix

Included:
   - Fix very old issue #232
   - Fix lack of `strtold` on cygwin (as reported in various web pages about compiling Redis under cygwin)

Redis can now compile under Cygwin without any end-user patches.

(Also see everybody having the same problems at https://www.google.com/search?q=redis+cygwin+SA_ONSTACK)