CI complains about -Wvla flag

[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’

Originally posted by @ben-schwen in #6274 (comment)

[MichaelChirico]

Could you link the exact failure? Is that R CMD check complaining?

[ben-schwen]

Yes it is in R CMD check

https://gitlab.com/Rdatatable/data.table/-/jobs/7447310770#L107
https://gitlab.com/Rdatatable/data.table/-/jobs/7447310761#L107
https://gitlab.com/Rdatatable/data.table/-/jobs/7447310746#L100

[MichaelChirico]

I think we can suppress this in our CI with _R_CHECK_COMPILATION_FLAGS_KNOWN_="-Wvla":

https://github.com/r-devel/r-svn/blob/83873fe7c1e3e23ef1ff4a986954bf1c8f01ce1d/src/library/tools/R/check.R#L3739C39-L3739C72

Since we're not setting that in Makevars or anything, CRAN will be none the wiser.