Different meanings of "debug" variant

[Blair Zajac]

I've seen two different styles of the debug variant:

- Compile everything with -g -O0
   libxml2 is this style

- Add additional binaries and/or libraries with debugging support
   boost is this style

I added "debug" to my variants.conf to get the second kind, not the first, but 
just discovered my libxml2 was compiled without optimization.

Should we have a distinction between the two and a standard naming convention 
for both?

Regards,
Blair