lib: silence clang/gcc -Wvla warnings in brotli headers

[vszakats]

brotli v1.0.0 throughout current latest v1.0.9 and latest master ¹ trigger this warning.

llvm/clang:

In file included from ./curl/lib/content_encoding.c:36:
./brotli/x64-ucrt/usr/include/brotli/decode.h:204:34: warning: variable length array used [-Wvla]
    const uint8_t encoded_buffer[BROTLI_ARRAY_PARAM(encoded_size)],
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./brotli/x64-ucrt/usr/include/brotli/port.h:253:34: note: expanded from macro 'BROTLI_ARRAY_PARAM'
#define BROTLI_ARRAY_PARAM(name) (name)
                                 ^~~~~~
In file included from ./curl/lib/content_encoding.c:36:
./brotli/x64-ucrt/usr/include/brotli/decode.h:206:48: warning: variable length array used [-Wvla]
    uint8_t decoded_buffer[BROTLI_ARRAY_PARAM(*decoded_size)]);
                           ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
./brotli/x64-ucrt/usr/include/brotli/port.h:253:35: note: expanded from macro 'BROTLI_ARRAY_PARAM'
#define BROTLI_ARRAY_PARAM(name) (name)
                                 ~^~~~~

gcc:

In file included from ./curl/lib/content_encoding.c:36:
./brotli/x64-ucrt/usr/include/brotli/decode.h:204:5: warning: ISO C90 forbids variable length array 'encoded_buffer' [-Wvla]
  204 |     const uint8_t encoded_buffer[BROTLI_ARRAY_PARAM(encoded_size)],
      |     ^~~~~
./brotli/x64-ucrt/usr/include/brotli/decode.h:206:5: warning: ISO C90 forbids variable length array 'decoded_buffer' [-Wvla]
  206 |     uint8_t decoded_buffer[BROTLI_ARRAY_PARAM(*decoded_size)]);
      |     ^~~~~~~

Closes #10738

Footnotes

1. https://github.com/google/brotli/commit/ed1995b6bda19244070ab5d331111f16f67c8054 ↩

[bagder]

I believe we generally avoid this warning in configure builds thanks to the convert -I options to -isystem macro.

[vszakats]

Aha, that's why it never appeared there, neat. This can work with GNU Make, but not sure how teach it to CMake.

[MarcelRaad]

not sure how teach it to CMake

By giving the SYSTEM option to include_directories. This is documented since at least CMake 3.0:
https://cmake.org/cmake/help/v3.0/command/include_directories.html

[vszakats]

@MarcelRaad: Thank you, noted the trick!

[ But.. I'd prefer to do this in the source. It's easier than syncing behaviour between build systems. Unless there is an objection! ]