build: don't check for protected visibility
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
There are not any places I can see where protected visibility is used so I opted for a removal rather than configuring around the issue.
Found as part of https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/issues/1132