It's not supported on macOS' clang compiler and will fail the visibility
check and thus make the G_GNUC_INTERNAL attribute do nothing.

Compiler stderr:
 /var/folders/nt/j2v2x4wd5cl33fq27mm31mwc0000gn/T/tmpxxf2zzi_/testfile.c:13:19: error: target does not support 'protected' visibility; using 'default' [-Werror,-Wunsupported-visibility]
  __attribute__ ((visibility ("protected")))
                  ^
1 error generated.

Checking if "GNU C visibility attributes test" compiles: NO