Allow building with native-tls

Right now we only support building with Rustls. However, there are quite
a few architectures that aren't yet supported by "ring" (the crypto
library used by Rustls), for example MIPS, PowerPC or SPARC.

To offer an alternative, I added the "native-tls" feature that can be
used instead of "native-roots". When used, the native TLS stack is used
instead of Rustls (i.e. SChannel on Windows, Secure Transport on macOS
and OpenSSL otherwise).

Cargo.toml

@@ -46,9 +46,20 @@ filetime = "0.2.10"
 
 [features]
 default = ["native-roots"]
+logging = ["env_logger"]
+
+# Reqwest (the HTTP client library) can handle TLS connections in three
+# different modes:
+#
+# - Rustls with native roots
+# - Rustls with WebPK roots
+# - Native TLS (SChannel on Windows, Secure Transport on macOS and OpenSSL otherwise)
+#
+# Exactly one of the three variants must be selected. By default, Rustls with
+# native roots is enabled.
 native-roots = ["reqwest/rustls-tls-native-roots"]
 webpki-roots = ["reqwest/rustls-tls-webpki-roots"]
-logging = ["env_logger"]
+native-tls = ["reqwest/native-tls"]
 
 [profile.release]
 lto = true

src/main.rs

@@ -18,10 +18,16 @@
 
 #[cfg(any(
     all(feature = "native-roots", feature = "webpki-roots"),
-    not(any(feature = "native-roots", feature = "webpki-roots")),
+    all(feature = "native-roots", feature = "native-tls"),
+    all(feature = "webpki-roots", feature = "native-tls"),
+    not(any(
+        feature = "native-roots",
+        feature = "webpki-roots",
+        feature = "native-tls"
+    )),
 ))]
 compile_error!(
-    "exactly one of feature \"native-roots\" and feature \"webpki-roots\" must be enabled"
+    "exactly one of the features \"native-roots\", \"webpki-roots\" or \"native-tls\" must be enabled"
 );
 
 use std::{env, process};