configure.ac: split -fvisibility and __attribute__((visibility)) checks

The former does not imply the latter and vice-versa. One such example is
the Sun compiler.

v2: Add missing closing brakets. (Alan)

Cc: Alan Coopersmith <alan.coopersmith@oracle.com>
Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>