Improve conditional build

[eddelbuettel]

The change in #406 works but is a little crude and not too self-explanatory. A nice approach might be to

• use configure as right now to determine 'yay' or 'nay' regarding blp support availability
• pass the 'yay or nay' state to the Makefile that is src/Makevars
• use conditional code in the Makefile to differentiate the two states

As added bonus we can then also clearly define a compile-time preprocessor variable indicating the 'yes we do' state rather than relying on the current double negative.