Explicitly request -Wvla to prevent backsliding

[MichaelChirico]

Follow-up to #6270.

IIUC, per here, we could also make this by supplying -pedantic-errors (instead of/in addition to -pedantic which we do already?).

I tried to find similar docs for clang/llvm but IME finding docs for compiler warnings is rarely fruitful...

[ben-schwen]

LGTM, but we should remove VLA before as in e.g.

data.table/src/forder.c

Line 915 in a01f00f

int TMP[my_n]; // on stack fine since my_n is very small (<=256)

data.table/src/forder.c

Line 1214 in a01f00f

int my_gs[ngrp];

[MichaelChirico]

Good point. There were a few others too. Now passing R CMD check under -Wvla. PTAL -- in particular on how TMP is free()'d in forder, I wasn't 100% confident there, could use the extra eyes.

[github-actions]

Generated via commit 712d64f

Download link for the artifact containing the test results: ↓ atime-results.zip

Time taken to finish the standard R installation steps: 11 minutes and 49 seconds

Time taken to run atime::atime_pkg on the tests: 3 minutes and 20 seconds

[ben-schwen]

CI is now complaining about these flags in:

• lin-dev-clang-cran
• lin-dev-gcc-strict-cran
• lin-rel-cran

Compilation used the following non-portable flag(s):
  ‘-Wvla’