[Rd] R CMD check warning about compiler warning flags

[Winston Chang]

On recent builds of R-devel, R CMD check gives a WARNING when some
compiler warning flags are detected, such as -Werror, because they are
non-portable. This appears to have been added in this commit:
  https://github.com/wch/r-source/commit/2e80059

I'm working on a package where these compiler warning flags are
present in a Makefile generated by a configure script -- that is, the
configure script detects whether the compiler supports these flags,
and if so, puts them in the Makefile. (The configure script is for a
third-party C library which is in a subdirectory of src/.)

Because the flags are added only if the system supports them, there
shouldn't be any worries about portability in practice.

Is there a way to get R CMD check to not raise warnings in cases like
this? I know I could modify the C library's configure.ac (which is
used to generate the configure script) but I'd prefer to leave the
library's code untouched if possible.

-Winston