restore hidden-visibility testing with Xcode 6.2

Xcode 6.2 (OS X 10.9.5) has a Clang version that is based on Clang 3.5 but does
not yet support -Wdate-time . Commit 7af9f8e2b6eb235923a329e8ff24012883fb6b86
introduced a regression where this compiler failed to pass the visibility tests
because it prints a warning about -Wdate-time. This in turn leads to linker
errors.