[msvc] Add a definition for _WIN32_WINNT to MonoPosixHelper

MonoPosixHelper fails to build in my testing, due to config.h bailing the
build out if it doesn't think the build platform is XP SP2 or higher.
Adding this define forces that minimum Windows version, allowing mono.sln
to build fully.

(cherry picked from commit 076e80f725d16d85d74e8aecbe281af53ebc62c2)
[msvc] Bump winconfig.h VERSION to match configure.ac
Unhelpfully, winconfig.h is used for building eglib, and it includes a hardcoded VERSION definition which needs to match configure.ac - otherwise msvc builds identify themselves as 2.11
[msvc] Define SSIZE_MAX if not already defined (i.e. on Windows)

limits.h from MSVC does not include SIZE_MAX or SSIZE_MAX. We need the latter to build sgen, so define it.

(cherry picked from commit cf4d77d21d2d15f5775a1cf72bfe8486e5a0dff5)
[msvc] Refreshed .def files (needed for SGen to build)

(cherry picked from commit 0279e7dc28c2d25ebe2fb87289b78c891ff84a15)
[msvc] Add missing sgen source files to libmonoruntime.vcxproj

(cherry picked from commit ab61b5c7ad106187b00be32bc356e2e197467745)
[msvc] Add _SGen Configurations to {eglib,libgc,genmdesc}.vcxproj

These configurations are needed to build a Debug_SGen version of mono.vcxproj, which is needed to build mono-sgen.exe

(cherry picked from commit d2b8c8557b90dfaef3d3b582a847fda12630c680)
[msvc] Fix MONO_INTERNAL undefined in Boehm

(cherry picked from commit 9f6f6a9ccef9e65adc281b5c0609f79d1dce6fcf)
[msvc] Allow cygwin "make" to be called after winsetup.bat

As-is, winsetup.bat replaces config.h with winconfig.h, which is needed for the MSVC compilation - but breaks all further use of cygwin's make (e.g. to build the class library, or run "make install").

This patch introduces a new cygconfig.h as a backup prior to squashing config.h (if it exists, so the runtime can still be built without cygwin ever being used), and makes winconfig.h use it on non-MSVC compilers (i.e. cygwin)

(cherry picked from commit 1d6b8a75fa31d60e9ddbc0b9eddbf05ed3fb6c12)
[msvc] duplicate 1d6b8a75fa31d60e9ddbc0b9eddbf05ed3fb6c12 for eglib

(cherry picked from commit 9bdd77969a8f9057f3b46994135d39adb3ef216f)