Add missing version check.
Add visibility specifier for clang.

Because this is wrapped in a version check in the GNU.cmake, it
needs to be repeated here.
Exclude some platforms which don't have the visibility flag.

Confirmed from output from the GenerateExportHeader test or other
googling.
Merge topic 'VISIBILITY_PRESET-property' into next

26f3105 Exclude some platforms which don't have the visibility flag.
49bec4e Add visibility specifier for clang.
408cc20 Add missing version check.