Gate stdext::checked_array_iterator usage on _ITERATOR_DEBUG_LEVEL

[bryan-lipinski]

Fix for #1069

checked_array_iterator is a Microsoft extension to the C++ Standard Library. In order to build cpprestsdk using other libraries such as libc++, the extension needs to be gated with a Microsoft specific macro. In this case _ITERATOR_DEBUG_LEVEL is the preferred macro for using checked iterators (As _SECURE_SCL is marked deprecated) .

I've validated this builds using MSVC and clang.cl.exe/libc++.

Note: The CMake file sets _SCL_SECURE_NO_WARNINGS which disables the warning that the code in question is attempting to fix.

elseif(WIN32)
add_definitions(-DUNICODE -D_UNICODE -DWIN32 -D_SCL_SECURE_NO_WARNINGS)

[BillyONeal]

The one Pipelines failure was a spurious network issue. Looks good to me, thanks for your contribution!