[libpng16] Fixed '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/*.dfa files that can be used in automatic build
scripts to ensure that these configurations continue to build.
[libpng16] Enabled WRITE_INVERT and WRITE_PACK in contrib/pngminim/encoder.
[libpng16] Update contrib/tools/pngfix.c
[libpng16] Updated "last changed" dates
Fix pngvalid 'fail' function declaration on the Intel C Compiler
This reverts to the previous 'static' implementation and works round the 'unused
static function' warning by using PNG_UNUSED.

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