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 by marking the entrypointFunctions array as used,
using a compiler attribute.

Signed-off-by: Charlotte Delenk <darkkirb@darkkirb.de>

src/GLX/glvnd_genentry.c

@@ -29,6 +29,7 @@
 
 #include "glvnd_genentry.h"
 #include "utils_misc.h"
+#include "compiler.h"
 
 #include <string.h>
 #include <stdint.h>
@@ -45,7 +46,7 @@
 #define GLX_STUBS_COUNT
 #include "g_glx_dispatch_stub_list.h"
 
-static GLVNDentrypointStub entrypointFunctions[GENERATED_ENTRYPOINT_MAX];
+USED static GLVNDentrypointStub entrypointFunctions[GENERATED_ENTRYPOINT_MAX];
 static char *entrypointNames[GENERATED_ENTRYPOINT_MAX] = {};
 static int entrypointCount = 0;