directfb: bump version, unbreak png support

The recent libpng bump broke png support in directfb, as it relied
on the legacy 1.2 interface. Fix it by bumping the directfb version,
and at the same time ensure that configure finds our cross-libpng-config
rather than what the host version the user might have in the path.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>