Add CMake build of SwiftPM dependencies

[jmschonfeld]

Building this project as a SwiftPM package on Windows requires first building a few dependencies as CMake projects (zlib, libxml, and curl). On Linux this is not required because these projects ship with the OS, but on Windows they don't ship with the OS or the toolchain. Normally, you must build each project manually with CMake and point the SwiftPM build at the build products via environment variables. This adds a new CMake option to the swift-corelibs-foundation project that, instead of building SCL-F itself via CMake, it checks out and builds these 3 dependencies via CMake and outputs the commands to run to set the appropriate environment variables. This CMake option can be triggered via a slightly modified version of the normal CMake build command:

cmake -G Ninja -B <build folder>
cmake --build <build folder> --target WindowsSwiftPMDependencies

At the end, the command will print and output like the following:

[0/1] Install the project...
[16/16] Running utility command for WindowsSwiftPMDependencies
Please set the following environment variables for the SwiftPM build:

LIBXML_INCLUDE_PATH=C:/Users/jmschonfeld/Documents/Repos/swift-corelibs-foundation/.build-cmake/windows-deps/libxml/include/libxml2
LIBXML_LIBRARY_PATH=C:/Users/jmschonfeld/Documents/Repos/swift-corelibs-foundation/.build-cmake/windows-deps/libxml/lib
CURL_INCLUDE_PATH=C:/Users/jmschonfeld/Documents/Repos/swift-corelibs-foundation/.build-cmake/windows-deps/curl/include
CURL_LIBRARY_PATH=C:/Users/jmschonfeld/Documents/Repos/swift-corelibs-foundation/.build-cmake/windows-deps/curl/lib
ZLIB_LIBRARY_PATH=C:/Users/jmschonfeld/Documents/Repos/swift-corelibs-foundation/.build-cmake/windows-deps/zlib/lib

After running the set commands, you can run swift test in this repo on Windows and it will build successfully.

[jmschonfeld]

@swift-ci please test

[jmschonfeld]

@swift-ci please test

[jmschonfeld]

@swift-ci please test Windows platform

[jmschonfeld]

@swift-ci please test

[jmschonfeld]

@swift-ci please test Windows platform