[libpng16] Changed warning message "is too large" to "may be too large"

in png_check_IHDR().
[libpng16] Updated warning message in png_check_IHDR()
[libpng16] Only warn about image too wide if warning not already issued.
[libpng16]Revised png_check_IHDR() to use PNG_SIZE_MAX instead of

PNG_UINT_32_MAX in the test for potential overflow in PNG_ROWBYTES.
[libpng16] Removed potentially misleading warning from png_check_IHDR().
[libpng16] Imported from libpng-1.6.9beta03.tar
[libpng16] Bump version to 1.6.9beta04
[libpng16] Updated scripts/makefile.* to use CPPFLAGS (Cosmin)
[libpng16] Added clang attribute support (Cosmin).
[libpng16] Correct a new comment in pngconf.h
[libpng16] Imported from libpng-1.6.9beta04.tar