build: do not print the default value of --c++-standard in help output #577

Job	Run time
Test with DPDK enabled / test	11m 14s
Test (clang++, C++20, dev) / test	4m 23s
Test with C++20 modules enabled / test	11m 50s
Test (clang++, C++20, debug) / test	6m 55s
Test (clang++, C++20, release) / test	4m 50s
Test (clang++, C++23, debug) / test	6m 52s
Test (clang++, C++23, dev) / test	4m 18s
Test (clang++, C++23, release) / test	15m 51s
Test (g++, C++20, debug) / test	7m 42s
Test (g++, C++20, release) / test	5m 12s
Test (g++, C++20, dev) / test	4m 14s
Test (g++, C++23, debug) / test	7m 27s
Test (g++, C++23, release) / test	5m 11s
Test (g++, C++23, dev) / test	4m 12s
	1h 40m 11s

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