GCC 4.0 not only added a visibility attribute, but also
a pragma to set it for a whole region of code.*
This commit exposes this via macros.

*: See https://gcc.gnu.org/gcc-4.0/changes.htmlReviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>