Fix alignment of bth_nonleaf_entry under 32-bit Mac GCC

The layout constraints on this structure changed in Subversion r48798,
requiring the overall structure to be 6 bytes in size.  This proves
somewhat tricky to get working on Mac GCC:

  1) We need to remove the PSTSDK_MS_STRUCT option, which overrides
     all other packing options.
  2) We need to add some new declarations to the 'page' member
     variable.
  3) We can no longer pass entry.page directly to make_pair, because
     make_pair requires an aligned reference for its second argument.
     Instead, we copy 'page' to a normally-aligned temporary variable.

With these changes, the code builds and passes all test suites again.

pstsdk/disk/disk.h

@@ -1341,12 +1341,14 @@ struct bth_header
 template<typename K>
 struct bth_nonleaf_entry
 {
-    K key;        //!< Key of the lower level page
-    heap_id page; //!< Heap id of the lower level page
-} PSTSDK_MS_STRUCT;
+    K key;                       //!< Key of the lower level page
+    heap_id page PSTSDK_PACK_2;  //!< Heap id of the lower level page
+};
 #pragma pack()
 //! \cond static_asserts
 static_assert(sizeof(bth_nonleaf_entry<ushort>) == 6, "bth_nonleaf_entry<ushort> incorrect size");
+static_assert(offsetof(bth_nonleaf_entry<ushort>, key) == 0, "bth_nonleaf_entry<ushort> key at incorrect offset");
+static_assert(offsetof(bth_nonleaf_entry<ushort>, page) == 2, "bth_nonleaf_entry<ushort> page at incorrect offset");
 //! \endcond
 
 //! \brief Entries which make up a "leaf" BTH allocation

pstsdk/ltp/heap.h

@@ -468,7 +468,8 @@ inline std::tr1::shared_ptr<pstsdk::bth_nonleaf_node<K,V> > pstsdk::bth_node<K,V
 
     for(uint i = 0; i < num_entries; ++i)
     {
-        child_nodes.push_back(std::make_pair(pbth_nonleaf_node->entries[i].key, pbth_nonleaf_node->entries[i].page));
+        heap_id page = pbth_nonleaf_node->entries[i].page;
+        child_nodes.push_back(std::make_pair(pbth_nonleaf_node->entries[i].key, page));
     }
 
 #ifndef BOOST_NO_RVALUE_REFERENCES

pstsdk/util/primitives.h

@@ -53,8 +53,10 @@
 // '#pragma ms_struct on', but it fails on at least some Linux systems.
 #ifdef __GNUC__
 #define PSTSDK_MS_STRUCT __attribute__((ms_struct))
+#define PSTSDK_PACK_2 __attribute__((packed, aligned(2)))
 #else
 #define PSTSDK_MS_STRUCT
+#define PSTSDK_PACK_2
 #endif
 
 namespace pstsdk