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
12 jobs for visibility-protected in 28 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Style Check
passed #518611
allowed to fail
style-check-diff

00:00:45

 
  Build
passed #518615
cross-android_api21_arm64

00:01:17

passed #518616
cross-android_api28_arm64

00:02:01

passed #518617
cross-mingw64

00:02:03

passed #518613
debian-stable-x86_64

00:03:35

passed #518612
fedora-x86_64

00:05:49

passed #518614
G_DISABLE_ASSERT

00:03:01

passed #518618
win32
msys2-mingw32

00:12:35

passed #518619
win32
vs2017-x64

00:08:53

 
  Coverage
passed #518620
coverage

00:01:08

77.9%
 
  Analysis
passed #518622
scan-build

00:13:57

failed #518621
allowed to fail
valgrind

00:00:43

 
Name Stage Failure
failed
valgrind Analysis
Uploading artifacts...
_build/config.h: found 1 matching files
_build/glib/glibconfig.h: found 1 matching files

_build/meson-logs: found 5 matching files

Uploading artifacts to coordinator... ok
id=518621 responseStatus=201 Created token=SkLHr1jA
Uploading artifacts...
WARNING: _build/valgrind-report.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1