Consult libpng-config more consistently.
This is mainly for simplicity, but it should also avoid some future problems like the ones we recently had with NetBSD. * configure.ac (LIBPNG): Configure after LIBZ. Use libpng-config for cflags, too. Append -lz if we're not already doing that with LIBZ. Do not bother appending -lm, since we always append that. Coalesce some duplicate code. * src/Makefile.in (PNG_CFLAGS): New var. (ALL_CFLAGS): Use it. * src/image.c [HAVE_PNG]: Don't worry about <libpng/png.h>, as CFLAGS now handles this. Fixes: debbugs:17339
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