Mark entrypointFunctions as used for clang+lto
LLVM currently ignores inline assembly references to symbols during dead code and data elimination while linking with link-time optimizations enabled.
This manifests in a very large amount of unresolved references to entrypointFunctions when building libglvnd using clang with LTO enabled.
This issue fixes #220 (closed) by marking the entrypointFunctions array as used, using a compiler attribute.
Signed-off-by: Charlotte Delenk darkkirb@darkkirb.de