boost/boost/graph/graph_concepts.hpp 1.36 doesn't compile

[conanbrink]

The change between revision 1.35 and 1.36 of 
boost/boost/graph/graph_concepts.hpp included the 
separation of a preprocessor #if statement into three 
lines without the backslashes that would normally 
indicate continuation of the first and second lines into 
the second and third.  This occurs at lines 163-165 of 
revision 1.36 of this file.

g++ 3.2 gets very grumpy about line 164, since that line 
made it through the preprocessor intact, when it should 
instead have been used as part of the decision made on 
line 163.

A couple of backslashes, one on line 163 and one on line 
164, solve the problem nicely.