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.

include/boost/core/enable_if.hpp

@@ -23,6 +23,11 @@
 
 namespace boost
 {
+  template<typename T, typename R=void>
+  struct enable_if_type
+  {
+    typedef R type;
+  };
  
   template <bool B, class T = void>
   struct enable_if_c {
@@ -80,6 +85,10 @@ namespace boost {
   template <typename T>
   struct enable_if_does_not_work_on_this_compiler;
 
+  template<typename T, typename R=void>
+  struct enable_if_type : enable_if_does_not_work_on_this_compiler<T>
+  { };
+
   template <bool B, class T = detail::enable_if_default_T>
   struct enable_if_c : enable_if_does_not_work_on_this_compiler<T>
   { };