Check that lazy_ostream argument is printable

The operator() is not instantiated till later, which makes it
difficult to locate an erroneous usage of <<.  Place a
static_assert in the ctor (copied from print_helper.hpp) to check
at construction time, which will point back to the << site.

include/boost/test/utils/lazy_ostream.hpp

@@ -15,6 +15,10 @@
 #include <boost/test/detail/config.hpp>
 #include <boost/test/tools/detail/print_helper.hpp>
 
+// Boost
+#include <boost/static_assert.hpp>
+#include <boost/type_traits/has_left_shift.hpp>
+
 // STL
 #include <iosfwd>
 
@@ -75,6 +79,8 @@ class lazy_ostream_impl : public lazy_ostream {
     , m_prev( prev )
     , m_value( value )
     {
+        BOOST_STATIC_ASSERT_MSG( (boost::has_left_shift<std::ostream,T>::value),
+                                "Type has to implement operator<< to be printable");
     }
 
     std::ostream&   operator()( std::ostream& ostr ) const BOOST_OVERRIDE