template specialization in C++03 as standard specify it.
This also works on GCC 4.6, 4.8.2 and 4.9 in C++11 and C++03, I needlessly
added this during some debugging where the compiler was telling me to do so.
I shouldn't have followed it's advice, as this breaks compatibility with
other compilers.
attribute_type and attribute_const_type when type is deduced can be
different than when the type is provided.
Indeed when specifying attribute_type and attribute_const_type manually
it's possible to provide a type which isn't const qualified as
attribute_const_type. When deducing the types from the get_expr, a const
and a non const qualified type is taken respectively for attribute_type
and attribute_const_type.
Indeed when some parameter are BOOST_PP_EMPTY the last parameter is
accessed correctly but results in a compilation error which doesn't stop
some compiler (e.g. g++) to go through the preprocessing pass and still
compile the binary correctly. But to avoid the error message I used a
workaround which behaves better. I preferred factoring it out to make
clear that there are some reason why this element is strangely accessed.
