Disable features exclusive to newer OpenSSL when using LibreSSL

Some features added to newer OpenSSL versions are absent in LibreSSL,
so don't mark these as present if LIBRESSL_VERSION_NUMBER is defined.
diff --git a/src/cryptography/hazmat/bindings/openssl/x509_vfy.py b/src/cryptography/hazmat/bindings/openssl/x509_vfy.py
index 6f05f4d..1f75b86 100644
--- a/src/cryptography/hazmat/bindings/openssl/x509_vfy.py
+++ b/src/cryptography/hazmat/bindings/openssl/x509_vfy.py
@@ -191,7 +191,7 @@
 
 CUSTOMIZATIONS = """
 /* OpenSSL 1.0.2+ verification error codes */
-#if OPENSSL_VERSION_NUMBER >= 0x10002000L
+#if OPENSSL_VERSION_NUMBER >= 0x10002000L && !defined(LIBRESSL_VERSION_NUMBER)
 static const long Cryptography_HAS_102_VERIFICATION_ERROR_CODES = 1;
 #else
 static const long Cryptography_HAS_102_VERIFICATION_ERROR_CODES = 0;
@@ -207,7 +207,7 @@
 #endif
 
 /* OpenSSL 1.0.2+ verification parameters */
-#if OPENSSL_VERSION_NUMBER >= 0x10002000L
+#if OPENSSL_VERSION_NUMBER >= 0x10002000L && !defined(LIBRESSL_VERSION_NUMBER)
 static const long Cryptography_HAS_102_VERIFICATION_PARAMS = 1;
 #else
 static const long Cryptography_HAS_102_VERIFICATION_PARAMS = 0;