Issue Description:

When I invoke build-script with extra-cmake-options they are not forwarded to the CoreFoundation build. This is due to the use of ExternalProject_Add to build CoreFoundation rather than add_subdirectory. I could whitelist the flags I'm trying to pass through (CURL_LIBRARY AND CURL_INCLUDE_DIR) but there's a high risk there are other important flags controlling the CoreFoundation build. Using add_subdirectory instead would allow CoreFoundation and Foundation to share a cache, so that important build settings don't vary between them.