We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
Fix MPEG-4 and VP8 APIs to export their external symbols as pure C
symbols, i.e. un-mangled for C++.
https://bugzilla.gnome.org/show_bug.cgi?id=731522
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>