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

Index: vendor/openssl-sys-0.9.102/build/main.rs
--- vendor/openssl-sys-0.9.102/build/main.rs.orig
+++ vendor/openssl-sys-0.9.102/build/main.rs
@@ -304,7 +304,7 @@ See rust-openssl documentation for more information:
             (3, 8, _) => ('3', '8', 'x'),
             (3, 9, 0) => ('3', '9', '0'),
             (3, 9, _) => ('3', '9', 'x'),
-            _ => version_error(),
+            _ => ('4', '0', 'x'),
         };
 
         println!("cargo:libressl=true");
