The LibreSSL version in -current isn't explicitly supported.

Index: .cargo/vendor/openssl-sys/build/main.rs
--- .cargo/vendor/openssl-sys/build/main.rs.orig
+++ .cargo/vendor/openssl-sys/build/main.rs
@@ -451,6 +451,7 @@ See rust-openssl documentation for more information:
             (4, 0, _) => ('4', '0', 'x'),
             (4, 1, 0) => ('4', '1', '0'),
             (4, 1, _) => ('4', '1', 'x'),
+            (4, _, _) => ('4', '1', 'x'),
             _ => version_error(),
         };
 
