in pngrutil.c

-> CPPFLAGS and PNG_USR_CONFIG -> PNG_USER_CONFIG).

and minor editing of contrib/arm-neon/README and contrib/examples/*.c

after IDAT with the sequential reader.

contrib/examples/*.c

but don't reject them and don't issue a copyright violation warning.
Simplified code in pngrutil.c that decides whether to warn about an
incorrect ancillary PLTE chunk.

png_push_read_chunk after recognizing the IDAT chunk, which avoids an
infinite loop while reading a datastream whose first IDAT chunk is of
zero-length.

instead of the default.

support older Clang versions (Jeremy Sequoia).

to run all the png_handle_*() and depend on them to return if info_ptr == NULL.
This improves the performance of png_read_end(png_ptr, NULL) and makes
it more robust against future programming errors.

implementation of the ARM NEON discovery function to be set at
build-time and provide sample implementations from the current code in the
contrib/arm-neon subdirectory. The __linux__ code has also been changed to
compile and link on Android by using /proc/cpuinfo, and the old linux code
is in contrib/arm-neon/linux-auxv.c.  The new code avoids POSIX and Linux
dependencies apart from opening /proc/cpuinfo and is C90 compliant.

and png_chunk_warning() from pngpriv.h to png.h to make them visible
to libpng-calling applications.