Do not redefine a constant available in LibreSSL's headers.

Index: lib_pypy/_cffi_ssl/_cffi_src/openssl/dh.py
--- lib_pypy/_cffi_ssl/_cffi_src/openssl/dh.py.orig
+++ lib_pypy/_cffi_ssl/_cffi_src/openssl/dh.py
@@ -110,7 +110,7 @@ int DH_set0_key(DH *dh, BIGNUM *pub_key, BIGNUM *priv_
 }
 #endif
 
-#if CRYPTOGRAPHY_OPENSSL_LESS_THAN_110
+#if CRYPTOGRAPHY_OPENSSL_LESS_THAN_110 && !CRYPTOGRAPHY_LIBRESSL_27_OR_GREATER
 #ifndef DH_CHECK_Q_NOT_PRIME
 #define DH_CHECK_Q_NOT_PRIME            0x10
 #endif