LibreSSL doesn't have @SECLEVEL yet.

Index: src/check_nrpe.c
--- src/check_nrpe.c.orig
+++ src/check_nrpe.c
@@ -151,7 +151,7 @@ int main(int argc, char **argv)
 	if (timeout_return_code == -1)
 		timeout_return_code = STATE_CRITICAL;
 	if (sslprm.cipher_list[0] == '\0')
-#if OPENSSL_VERSION_NUMBER >= 0x10100000
+#if OPENSSL_VERSION_NUMBER >= 0x10100000 && !defined(LIBRESSL_VERSION_NUMBER)
 		strncpy(sslprm.cipher_list, "ALL:!MD5:@STRENGTH:@SECLEVEL=0", MAX_FILENAME_LENGTH - 1);
 #else
 		strncpy(sslprm.cipher_list, "ALL:!MD5:@STRENGTH", MAX_FILENAME_LENGTH - 1);
@@ -751,7 +751,7 @@ void usage(int result)
 		printf("                              TLSv1.2   TLS v1.2 only\n");
 		printf("                              TLSv1.2+  TLS v1.2 or above\n");
 		printf(" -L, --cipher-list=LIST       The list of SSL ciphers to use (currently defaults\n");
-#if OPENSSL_VERSION_NUMBER >= 0x10100000
+#if OPENSSL_VERSION_NUMBER >= 0x10100000 && !defined(LIBRESSL_VERSION_NUMBER)
 		printf("                              to \"ALL:!MD5:@STRENGTH:@SECLEVEL=0\". THIS WILL change in a future release.)\n");
 #else
 		printf("                              to \"ALL:!MD5:@STRENGTH\". THIS WILL change in a future release.)\n");
@@ -1011,7 +1011,7 @@ void setup_ssl()
 		} 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);
