My package Postal is best compiled with libssl095a-dev installed so that it
can support SSL. However it will detect the lack of such support at build
time and be compiled without SSL if the header files aren't installed.
The next version will have build-depends on libssl095a-dev, but I think that
it would be more accurate to have build-suggests. So that if you can get the
libssl095a-dev then you should do so, but my program will work quite well
(with reduced functionality) if you don't.