[boost] is_abstract update

From: John Maddock (john_at_[hidden])
Date: 2004-12-04 07:54:46

Following on from the recent discussion, we now have:

BOOST_NO_IS_ABSTRACT added to the config system, this is likely to cause 
some new failures with config_test.cpp until we get the macro added to those 
compiler config files that need it (although Gcc, Visual C++, Borland C++ 
are already taken care of).

is_abstract no longer uses SFINAE when BOOST_NO_IS_ABSTRACT is defined, 
instead it defaults to the same value as is_polymorphic: the rationale is 
that "true" is the safe default (if there is one), but since non-polymorphic 
types obviously can't be abstract, we can safely evaluate to false for 
those.

Users wanting some other behaviour should check for BOOST_NO_IS_ABSTRACT.

Robert, is this enough for you to fix the ODR problem in the serialisation 
lib for is_abstract?

John.