ustream-ssl-mbedtls: fix build on x86_64-darwin

pkgs/top-level/all-packages.nix

···
 
   ustream-ssl-wolfssl = callPackage ../development/libraries/ustream-ssl { ssl_implementation = wolfssl; additional_buildInputs = [ openssl ]; };
 
-  ustream-ssl-mbedtls = callPackage ../development/libraries/ustream-ssl { ssl_implementation = mbedtls_2; };
+  ustream-ssl-mbedtls = callPackage ../development/libraries/ustream-ssl {
+    ssl_implementation = mbedtls_2;
+    stdenv = if stdenv.isDarwin then overrideSDK stdenv "11.0" else stdenv;
+  };
 
   uri = callPackage ../development/libraries/uri { };