https://github.com/sfackler/rust-openssl/pull/2124

Index: vendor/openssl-0.10.57/build.rs
--- vendor/openssl-0.10.57/build.rs.orig
+++ vendor/openssl-0.10.57/build.rs
@@ -73,6 +73,9 @@ fn main() {
         for var in vars.split(',') {
             println!("cargo:rustc-cfg=osslconf=\"{}\"", var);
         }
+        if version >= 0x3_09_00_00_0 {
+            println!("cargo:rustc-cfg=libressl390");
+        }
     }
 
     if let Ok(version) = env::var("DEP_OPENSSL_VERSION_NUMBER") {