Always add -L${prefix}/lib, -I${prefix}/include?

[Ryan Schmidt]

Many ports have had to add -L{$prefix}/lib to the LDFLAGS and -I$ 
{prefix}/include to the CFLAGS. Wouldn't it be much easier if the  
MacPorts infrastructure would add that to the LDFLAGS and CFLAGS  
generally for all ports? Could that hurt anything?

I would think the same should apply to CPPFLAGS and/or CXXFLAGS, not  
that I have yet understood why there are two variables for C++ code  
or what the difference is between them.