Use `-std=gnu11` instead of `-std=c11` on Linux

[4og]

What

Baselibs module is going to depend on OpenSSL from BCR. The BCR-provided OpenSSL relies on GNU extensions for C11, and therefore requires compilation with -std=gnu11 on Linux.

At the moment, the Linux cc_toolchain enforces -std=c11, which causes the build to fail.
The task is to investigate the feasibility and impact of switching the Linux toolchain to use -std=gnu11 instead.

Acceptance Criteria (DoD)

How

Change here to gnu11

bazel_cpp_toolchains/templates/linux/cc_toolchain_flags.bzl.template

Lines 66 to 69 in 1f1b1d1

	# Default compile flags for C language when no specific build type is selected.
	DEFAULT_C_COMPILE_FLAGS = get_flag_group([
	"-std=c11",
	])

[4og]

@odra, @opajonk, @nradakovic, what do you think about this?