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

@@ -46,8 +46,12 @@ filetime = "0.2.10"
 
 [features]
 default = ["native-roots"]
+# Use Rustls with native roots
 native-roots = ["reqwest/rustls-tls-native-roots"]
+# Use Rustls with WebPK roots
 webpki-roots = ["reqwest/rustls-tls-webpki-roots"]
+# Use native TLS (SChannel on Windows, Secure Transport on macOS and OpenSSL otherwise)
+native-tls = ["reqwest/native-tls"]
 logging = ["env_logger"]
 
 [profile.release]

src/main.rs

@@ -17,11 +17,22 @@
 #![allow(clippy::too_many_lines)]
 
 #[cfg(any(
+    all(
+        feature = "native-roots",
+        feature = "webpki-roots",
+        feature = "native-tls"
+    ),
     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 features \"native-roots\", \"webpki-roots\" or \"native-tls\" must be enabled"
 );
 
 use std::{env, process};