Consult libpng-config more consistently.
* configure.ac (edit_cflags): Move this up and to the top level, so that PNG_CFLAGS can use it too. (PNG_CFLAGS): New var. (png_longjmp): Use PNG_CFLAGS when checking. (LIBPNG): Be consistent about -lpng16 etc; e.g., don't use -lpng16 in some places and -lpng in others. Test libpng-config's exit status. If it succeeds, use its output rather than appending -lz -lm. * 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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