Allow HANDLE_AS_UNKNOWN to work when other options are configured off
Also fix the pngminim makefiles to work when $(MAKEFLAGS) contains stuff which
terminates the make options (as by default in recent versions of Gentoo).

Signed-off-by: John Bowler <jbowler@acm.org>
[libpng16] Allow HANDLE_AS_UNKNOWN to work when other options are configured

off. Also fixed the pngminim makefiles to work when $(MAKEFLAGS) contains stuff
which terminates the make options (as by default in recent versions of Gentoo).
Merge branch 'libpng16' of /home/jbowler/src/libpng/libpng into libpng16