Modify compiler flags to suppress variable length array error

[peternewman]

Added -Wno-error=vla to COMMON_CXXFLAGS and COMMON_TESTING_FLAGS to prevent fatal warnings for variable-length arrays.

Plus other minor tidying.

[DaAwesomeP]

In the future I think we should move to std::vector or similar. VLAs are not supported by some platforms (I think Windows is one of these?).

[peternewman]

I've created #2053 to track that. TBH I'd assumed extracting from argc/argv would be a bit of a pain, but it doesn't look too bad thankfully.

[aroffringa]

Flag changes look fine. I added one suggestion for the cast for which you use NOLINT.

[aroffringa]

I don't think a static_cast should even compile here; the C cast will behave like a reinterpret_cast in this case (but is more permissive). I think best would be to do an explicit reinterpret_cast:

Suggested change

	// Changing these to a static_cast breaks the tests at least
	tr.tx_buf = (uint64_t) tx_buf; // NOLINT(readability/casting)
	tr.rx_buf = (uint64_t) rx_buf; // NOLINT(readability/casting)
	tr.tx_buf = reinterpret_cast<uint64_t>(tx_buf);
	tr.rx_buf = reinterpret_cast<uint64_t>(rx_buf);