[cpprestsdk] use platform expressions in default-features

[autoantwort]

This uses the new feature from microsoft/vcpkg-tool#813.

One "problem" is that people with old vcpkg-tool versions will get an error if they want to use the port and there is afaik not way to tell them that they have to update vcpkg-tool.

[jimwang118]

Install microsoft-signalr first and the following problems will occur when installing signalrclient.

Installing 1/1 signalrclient:x64-windows...
error: The following files are already installed in F:/cpprestsdk/installed/x64-windows and are in conflict with signalrclient:x64-windows
Installed by microsoft-signalr:x64-windows
include/signalrclient/_exports.h
    include/signalrclient/connection_state.h
    include/signalrclient/hub_connection.h
    include/signalrclient/hub_exception.h
    include/signalrclient/log_writer.h
    include/signalrclient/trace_level.h
    include/signalrclient/transport_type.h
    include/signalrclient/web_exception.h

Elapsed time to handle signalrclient:x64-windows: 13.6 ms

[jimwang118]

Note: I will be converting your PR to draft status. When you respond, please revert to "ready for review". That way, I can be aware that you've responded since you can't modify the tags.

[autoantwort]

Install microsoft-signalr first and the following problems will occur when installing signalrclient.

This is not new:

vcpkg/scripts/ci.baseline.txt

Lines 785 to 795 in 78b6158

	# Conflicts with signalrclient
	microsoft-signalr:arm64-windows=skip
	microsoft-signalr:arm-uwp=skip
	microsoft-signalr:x64-linux=skip
	microsoft-signalr:x64-osx=skip
	microsoft-signalr:arm64-osx=skip
	microsoft-signalr:x64-uwp=skip
	microsoft-signalr:x64-windows=skip
	microsoft-signalr:x64-windows-static=skip
	microsoft-signalr:x64-windows-static-md=skip
	microsoft-signalr:x86-windows=skip