Fix -Wundef warnings in core extensions

* See [Feature #17752]

ext/nkf/nkf-utf8/nkf.h

@@ -16,8 +16,8 @@
 #define X0201_DEFAULT TRUE
 #endif
 
-#if DEFAULT_NEWLINE == 0x0D0A
-#elif DEFAULT_NEWLINE == 0x0D
+#if defined(DEFAULT_NEWLINE) && DEFAULT_NEWLINE == 0x0D0A
+#elif defined(DEFAULT_NEWLINE) && DEFAULT_NEWLINE == 0x0D
 #else
 #define DEFAULT_NEWLINE 0x0A
 #endif

ext/openssl/ossl_pkey_ec.c

@@ -1469,7 +1469,7 @@ static VALUE ossl_ec_point_mul(int argc, VALUE *argv, VALUE self)
 	if (EC_POINT_mul(group, point_result, bn_g, point_self, bn, ossl_bn_ctx) != 1)
 	    ossl_raise(eEC_POINT, NULL);
     } else {
-#if OPENSSL_VERSION_MAJOR+0 >= 3 || defined(LIBRESSL_VERSION_NUMBER)
+#if (defined(OPENSSL_VERSION_MAJOR) && OPENSSL_VERSION_MAJOR >= 3) || defined(LIBRESSL_VERSION_NUMBER)
         rb_raise(rb_eNotImpError, "calling #mul with arrays is not" \
                  "supported by this OpenSSL version");
 #else