Link to custom libcurl on macOS

[jcamiel]

I'm trying to test HTTP3 with curl-rust on macOS, so I want to link against a custom version of libcurl (that supports HTTP3).

In

curl-rust/curl-sys/build.rs

Lines 18 to 19 in ff6ad21

	if cfg!(feature = "force-system-lib-on-osx") && target.contains("apple") {
	return println!("cargo:rustc-flags=-l curl");

there is a feature flag force-system-lib-on-osx that forces system libcurl linking.

Few lines after:

curl-rust/curl-sys/build.rs

Lines 26 to 28 in ff6ad21

	if target.contains("apple") && (!cfg!(feature = "http2") || curl_config_reports_http2()) {
	return println!("cargo:rustc-flags=-l curl");
	}

If target is macOS with system curl supporting HTTP2 (like recent version Ventura for instance), we force also system libcurl linking.

It seems to me that no matter force-system-lib-on-osx, we always link to system libcurl on macOS, which seems a bug to me.

Note: I successfully link to a custom libcurl supporting HTTP3 by removing

curl-rust/curl-sys/build.rs

Lines 26 to 28 in ff6ad21

	if target.contains("apple") && (!cfg!(feature = "http2") || curl_config_reports_http2()) {
	return println!("cargo:rustc-flags=-l curl");
	}

and relies on pkg-config code path.
For building a custom libcurl supporting HTTP3, I've used the brew formulae https://github.com/cloudflare/homebrew-cloudflare with brew install --HEAD -s curl.rb