cmake: fix build - RE_VA_ARGS compatibility

[cspiel1]

includes missed updates of CMakeLists.txt from baresip repo

[cspiel1]

Not clear for me why the clang tests fail. I'll check the differences in CMakeLists.txt.

[sreimers]

For clang these flags are needed:

  if(CMAKE_C_COMPILER_ID MATCHES "Clang")
    list(APPEND c_flags
      -Watomic-implicit-seq-cst
      -Wshorten-64-to-32
      -Wno-gnu-zero-variadic-macro-arguments
      -Wno-c2x-extensions
    )
  endif()

[cspiel1]

And, should I add _GNU_SOURCE to RE_DEFINITIONS?

[cspiel1]

@sreimers please could you review and merge?

edit: On my dev host could reproduce the clang build failure without the last 4 commits. Now it builds.

[cspiel1]

thanks!