Fix 'minimal' builds
Various obviously useful minimal configurations don't build because of missing
contrib/libtests test programs and overly complex dependencies in
scripts/pnglibconf.dfa.

This change adds contrib/conftest/*; test .dfa files that can be used in
automatic build scripts to ensure that these configurations continue to build.

Signed-off-by: John Bowler <jbowler@acm.org>