Fix flat_hash_map when used across library boundaries.

Now it works with dll. For more details see:
skarupke/flat_hash_map#26

Author: aleksisch

include/ska/flat_hash_map.hpp

@@ -164,12 +164,6 @@ struct sherwood_v3_entry
     ~sherwood_v3_entry()
     {
     }
-    static sherwood_v3_entry * empty_default_table()
-    {
-        static sherwood_v3_entry result[min_lookups] = { {}, {}, {}, {special_end_value} };
-        return result;
-    }
-
     bool has_value() const
     {
         return distance_from_desired >= 0;
@@ -755,13 +749,22 @@ class sherwood_v3_table : private EntryAlloc, private Hasher, private Equal
     }
 
 private:
-    EntryPointer entries = Entry::empty_default_table();
+    EntryPointer entries = empty_default_table();
     size_t num_slots_minus_one = 0;
     typename HashPolicySelector<ArgumentHash>::type hash_policy;
     int8_t max_lookups = detailv3::min_lookups - 1;
     float _max_load_factor = 0.5f;
     size_t num_elements = 0;
 
+    EntryPointer empty_default_table()
+    {
+        EntryPointer result = AllocatorTraits::allocate(*this, detailv3::min_lookups);
+        EntryPointer special_end_item = result + static_cast<ptrdiff_t>(detailv3::min_lookups - 1);
+        for (EntryPointer it = result; it != special_end_item; ++it)
+            it->distance_from_desired = -1;
+        special_end_item->distance_from_desired = Entry::special_end_value;
+        return result;
+    }
     static int8_t compute_max_lookups(size_t num_buckets)
     {
         int8_t desired = detailv3::log2(num_buckets);
@@ -841,16 +844,13 @@ class sherwood_v3_table : private EntryAlloc, private Hasher, private Equal
 
     void deallocate_data(EntryPointer begin, size_t num_slots_minus_one_, int8_t max_lookups_)
     {
-        if (begin != Entry::empty_default_table())
-        {
-            AllocatorTraits::deallocate(*this, begin, num_slots_minus_one_ + max_lookups_ + 1);
-        }
+        AllocatorTraits::deallocate(*this, begin, num_slots_minus_one_ + max_lookups_ + 1);
     }
 
     void reset_to_empty_state()
     {
         deallocate_data(entries, num_slots_minus_one, max_lookups);
-        entries = Entry::empty_default_table();
+        entries = empty_default_table();
         num_slots_minus_one = 0;
         hash_policy.reset();
         max_lookups = detailv3::min_lookups - 1;