Introduces enable_if_type

enable_if_type allow to perform SFINAE check on the existence
of a dependent type.

It has been used here and there in various boost library but it's
useful enough to warrant an autonomous existence.

doc/enable_if.qbk

@@ -304,6 +304,26 @@ depends on the template arguments of the class. Note that
 again, the second argument to `enable_if` is not needed; the
 default (`void`) is the correct value.
 
+The `enable_if_type` template is usable this scenario but instead of
+using a type traits to enable or disable a specialization, it use a
+SFINAE context to check for the existence of a dependent type inside
+its parameter. For example, the following structure extracts a dependent
+`value_type` from T if and only if `T::value_type` exists.
+
+``
+template <class T, class Enable = void>
+class value_type_from
+{
+  typedef T type;
+};
+
+template <class T>
+class value_type_from<T, typename enable_if_type<typename T::value_type>::type>
+{
+  typedef typename T::value_type type;
+};
+``
+
 [endsect]
 
 [section Overlapping enabler conditions]