gio: Check __APPLE__ in a few places instead of HAVE_COCOA

`HAVE_COCOA` should be used only in the places where we’re actually
depending on the Cocoa toolkit. It should not be used as a general way
of detecting building on a Darwin-based OS such as macOS.

Conversely, there are a few places in the code where we do want to
specifically detect the Cocoa toolkit (and others where we specifically
want to detect Carbon), so keep `HAVE_COCOA` and `HAVE_CARBON` around.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #2802

gio/tests/file.c

@@ -2472,7 +2472,7 @@ get_size_from_du (const gchar *path, guint64 *size)
   GError *error = NULL;
   gchar *du_path = NULL;
 
-#ifndef HAVE_COCOA
+#ifndef __APPLE__
   du_path = g_find_program_in_path ("du");
 #endif
 
@@ -3556,7 +3556,7 @@ test_query_zero_length_content_type (void)
                        NULL, &error);
   g_assert_no_error (error);
 
-#ifndef HAVE_COCOA
+#ifndef __APPLE__
   g_assert_cmpstr (g_file_info_get_content_type (file_info), ==, "text/plain");
 #else
   g_assert_cmpstr (g_file_info_get_content_type (file_info), ==, "public.text");