configure.ac: bump version to 2.4.56 for release
configure: Support symbol visibility when available

Checks whether or not the compiler supports the -fvisibility option. If
so it sets the VISIBILITY_CFLAGS variable which can be added to the per
directory AM_CFLAGS where appropriate.

By default all symbols will be hidden via the VISIBILITY_CFLAGS. The
drm_public macro can be used to mark symbols that should be exported.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
nouveau: Only export public functions.

This hides all the abi16_* functions and the nouveau_debug variable,
they should have been private to begin with.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
radeon: Use symbol visibility.

All the bof_* symbols are now no longer exported.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
intel: Use symbol visibility.

No exports changed for this driver.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>