cmAlgorithms: Add cmRemoveDuplicates algorithm.

Start by creating a vector to hold a unique values of the input range.  We
expect that in most cases, there will be relatively few duplicates, so
reserving enough memory for a complete copy is worthwhile.  Unlike a solution
involving a std::set, this algorithm allocates all the memory it needs
in one go and in one place, so it is more cache friendly.

Populate the unique copy with a lower_bound insert algorithm and record the
indices of duplicates.  This is the same complexity as the std::set insert
algorithm, but without the need to allocate memory on the heap and other
disadvantages of std::set.

Remove the duplicates with the cmRemoveIndices algorithm.

Source/cmAlgorithms.h

@@ -250,4 +250,33 @@ typename Range::const_iterator cmRemoveMatching(Range &r, MatchRange const& m)
                         ContainerAlgorithms::BinarySearcher<MatchRange>(m));
 }
 
+template<typename Range>
+typename Range::const_iterator cmRemoveDuplicates(Range& r)
+{
+  std::vector<typename Range::value_type> unique;
+  unique.reserve(r.size());
+  std::vector<size_t> indices;
+  size_t count = 0;
+  for(typename Range::const_iterator it = r.begin();
+      it != r.end(); ++it, ++count)
+    {
+    typename Range::iterator low =
+        std::lower_bound(unique.begin(), unique.end(), *it);
+    if (low == unique.end() || *low != *it)
+      {
+      unique.insert(low, *it);
+      }
+    else
+      {
+      indices.push_back(count);
+      }
+    }
+  if (indices.empty())
+    {
+    return r.end();
+    }
+  std::sort(indices.begin(), indices.end());
+  return cmRemoveIndices(r, indices);
+}
+
 #endif