Add GNU ext. flag for c compile action.

[nradakovic]

Some of the libraries that we depend has to be built with gnu ext, like libopenssl. This feature ensure we can switch between them.

Issue: #46

[opajonk]

I did a simple rebuilt of reference_integration main with

git_override(
    module_name = "score_bazel_cpp_toolchains",
    commit = "b70e1d26b9afe1c5b68ac1b00128df1ae25a3d80",
    remote = "https://github.com/eclipse-score/bazel_cpp_toolchains.git",
)

in bazel_common/score_gcc_toolchains.MODULE.bazel, via ./score_starter and choosing the EB Linux. I got no issues with it.

[odra]

Tried it in a build myself, worked ok too.