[Ada] GNAT.Sockets: fix socket timeout on recent Windows versions

[Ada] GNAT.Sockets: fix socket timeout on recent Windows versions

2019-07-04  Dmitriy Anisimkov  <anisimko@adacore.com>

gcc/ada/

	* doc/gnat_ugn/platform_specific_information.rst: Document
	Windows socket timeout particularity.
	* gnat_ugn.texi: Regenerate.
	* gsocket.h: Include versionhelpers.h.
	* socket.c (__gnat_minus_500ms): New function.
	* libgnat/g-sothco.ads (Minus_500ms_Windows_Timeout): New
	imported function.
	* libgnat/g-socket.adb (Set_Socket_Option): Refactor to remove
	500ms from the requested timeout only on old Windows version.

From-SVN: r273045