[3.8] bpo-40515: Require OPENSSL_THREADS (GH-19953) (GH-20119)

diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 43b236c..1da65ea 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -75,6 +75,10 @@
 #  endif
 #endif
 
+#ifndef OPENSSL_THREADS
+#  error "OPENSSL_THREADS is not defined, Python requires thread-safe OpenSSL"
+#endif
+
 /* SSL error object */
 static PyObject *PySSLErrorObject;
 static PyObject *PySSLCertVerificationErrorObject;
@@ -6008,7 +6012,7 @@
     if (!_setup_ssl_threads()) {
         return NULL;
     }
-#elif OPENSSL_VERSION_1_1 && defined(OPENSSL_THREADS)
+#elif OPENSSL_VERSION_1_1
     /* OpenSSL 1.1.0 builtin thread support is enabled */
     _ssl_locks_count++;
 #endif