Add support for compilation with libressl,
see https://github.com/krb5/krb5/pull/607

--- src/plugins/preauth/pkinit/pkinit_crypto_openssl.c
+++ src/plugins/preauth/pkinit/pkinit_crypto_openssl.c
@@ -191,7 +191,7 @@ pkinit_pkcs11_code_to_text(int err);
     (*_x509_pp) = PKCS7_cert_from_signer_info(_p7,_si)
 #endif
 
-#if OPENSSL_VERSION_NUMBER < 0x10100000L
+#if OPENSSL_VERSION_NUMBER < 0x10100000L || defined(LIBRESSL_VERSION_NUMBER)
 
 /* 1.1 standardizes constructor and destructor names, renaming
  * EVP_MD_CTX_{create,destroy} and deprecating ASN1_STRING_data. */
@@ -3059,7 +3059,7 @@ cleanup:
     return retval;
 }
 
-#if OPENSSL_VERSION_NUMBER >= 0x10100000L
+#if OPENSSL_VERSION_NUMBER >= 0x10100000L && !defined(LIBRESSL_VERSION_NUMBER)
 
 /*
  * We need to decode DomainParameters from RFC 3279 section 2.3.3.  We would
--- src/plugins/preauth/pkinit/pkinit_crypto_openssl.h
+++ src/plugins/preauth/pkinit/pkinit_crypto_openssl.h
@@ -46,7 +46,7 @@
 #include <openssl/asn1.h>
 #include <openssl/pem.h>
 
-#if OPENSSL_VERSION_NUMBER >= 0x10100000L
+#if OPENSSL_VERSION_NUMBER >= 0x10100000L && !defined(LIBRESSL_VERSION_NUMBER)
 #include <openssl/asn1t.h>
 #else
 #include <openssl/asn1_mac.h>