Also removed two obsolete comments from png.c

to catch library and header mismatch, also found at the end of pngtest.c.

as it was prior to libpng-1.2.16beta2, to simplify version differences.

so it will work with automake versions earlier than 1.11.1

to remove obsolete statement about a custom ltmain.sh

in printf statements in pngvalid.c, and changed %ud to %lu.

and re-removed mkinstalldirs.

that CCP writes to standard output by default (Martin Banky).

$AUTOHEADER, $AUTOPOINT, $ACLOCAL and $LIBTOOLIZE

Studio 2010 (John Bowler).

in pngpriv.h in case the user forgot to define them in their pngusr.h.  This
means it is safe for people not building a DLL to omit these defines from pngusr.h.