swift-corelibs uses libcurl to implement `NSURLSession` in Foundation
via the symbols exported by CF. Foundation is not build on Darwin, and
these symbols are not exported by the system CoreFoundation.
By not linking against libcurl, this breaks a cycle between CF and
libcurl. That should allow libcurl to drop the patch disabling
linking against the SystemConfiguration and restore NAT64 support.
Unfortunately, the Darwin stdenv bootstrap still needs to build
dependencies that use `fetchFromGitHub`. While it can drop curl from the
final stdenv, it still needs to use it during the stdenv bootstrap.
ebc1bcf409