LibreSSL doesn't have @SECLEVEL yet.

Index: src/nrpe.c
--- src/nrpe.c.orig
+++ src/nrpe.c
@@ -83,7 +83,7 @@ struct addrinfo *listen_addrs = NULL;
 int       listen_socks[MAX_LISTEN_SOCKS];
 char      remote_host[MAX_HOST_ADDRESS_LENGTH];
 char     *macro_argv[MAX_COMMAND_ARGUMENTS];
-char      config_file[MAX_INPUT_BUFFER] = "nrpe.cfg";
+char      config_file[MAX_INPUT_BUFFER] = "/etc/nrpe.cfg";
 char      server_address[NI_MAXHOST] = "";
 char     *command_name = NULL;
 int       log_facility = LOG_DAEMON;
@@ -147,7 +147,7 @@ struct _SSL_PARMS {
 	ClntCerts client_certs;
 	SslLogging log_opts;
 } sslprm = {
-#if OPENSSL_VERSION_NUMBER >= 0x10100000
+#if OPENSSL_VERSION_NUMBER >= 0x10100000 && !defined(LIBRESSL_VERSION_NUMBER)
 NULL, NULL, NULL, "ALL:!MD5:@STRENGTH:@SECLEVEL=0", TLSv1_plus, TRUE, 0, SSL_NoLogging
 #else
 NULL, NULL, NULL, "ALL:!MD5:@STRENGTH", TLSv1_plus, TRUE, 0, SSL_NoLogging
@@ -463,7 +463,7 @@ void init_ssl(void)
 	} else {
 		/* use anonymous DH ciphers */
 		if (sslprm.allowDH == 2) {
-#if OPENSSL_VERSION_NUMBER >= 0x10100000
+#if OPENSSL_VERSION_NUMBER >= 0x10100000 && !defined(LIBRESSL_VERSION_NUMBER)
 			strncpy(sslprm.cipher_list, "ADH@SECLEVEL=0", MAX_FILENAME_LENGTH - 1);
 #else
 			strncpy(sslprm.cipher_list, "ADH", MAX_FILENAME_LENGTH - 1);
