Try at fixing multiple-definition of NullPool

If we don't have visibility, make NullPool static.

src/hb-open-type-private.hh

@@ -134,7 +134,13 @@ static inline Type& StructAfter(TObject &X)
 
 #define HB_NULL_POOL_SIZE 264
 static_assert (HB_NULL_POOL_SIZE % sizeof (void *) == 0, "Align HB_NULL_POOL_SIZE.");
-extern HB_INTERNAL const void * const _hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)];
+
+#ifdef HB_NO_VISIBILITY
+static
+#else
+extern HB_INTERNAL
+#endif
+const void * const _hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)];
 
 /* Generic nul-content Null objects. */
 template <typename Type>

src/hb-ot-layout.cc

@@ -40,7 +40,9 @@
 #include "hb-ot-map-private.hh"
 
 
+#ifndef HB_NO_VISIBILITY
 const void * const OT::_hb_NullPool[HB_NULL_POOL_SIZE / sizeof (void *)] = {};
+#endif
 
 
 hb_ot_layout_t *

src/hb-private.hh

@@ -120,9 +120,7 @@ extern "C" void  hb_free_impl(void *ptr);
 #endif
 
 #ifndef HB_INTERNAL
-# ifdef __SUNPRO_CC
-#  define HB_INTERNAL __hidden
-# elif !defined(__MINGW32__) && !defined(__CYGWIN__)
+# if !defined(HB_NO_VISIBILITY) && !defined(__MINGW32__) && !defined(__CYGWIN__)
 #  define HB_INTERNAL __attribute__((__visibility__("hidden")))
 # else
 #  define HB_INTERNAL