Enabling SSE support

Copyright (c) 2016 Google, Inc.
Written by Mike Klein and Matt Sarett

To enable SSE support in libpng, apply intel_sse.patch in your build
directory.  If you prefer,  manually edit configure.ac and Makefile.am,
following the instructions below.

Then configure libpng with -DPNG_INTEL_SSE in CPPFLAGS.
If you only want to optimize 4bpp images, also use -DPNG_NO_INTEL_SSE_3BPP.

If you have moved intel_init.c and filter_sse2_intrinsics.c to a different
directory, be sure to update the '#include "../../pngpriv.h"' line in both
files if necessary to point to the correct relative location of pngpriv.h
with respect to the new location of those files.

1. Insert the following lines above the copyright line near the top of
configure.ac:

-----------------cut----------------
# Copyright (c) 2016 Google, Inc.
# Written by Mike Klein and Matt Sarett
# Derived from the ARM supporting code in libpng/configure.ac, which was
-----------------cut----------------

2. Add the following code to configure.ac under HOST SPECIFIC OPTIONS
directly beneath the section for ARM:

-----------------cut----------------
# INTEL
# =====
#
# INTEL SSE (SIMD) support.

AC_ARG_ENABLE([intel-sse],
   AS_HELP_STRING([[[--enable-intel-sse]]],
      [Enable Intel SSE optimizations: =no/off, yes/on:]
      [no/off: disable the optimizations;]
      [yes/on: enable the optimizations.]
      [If not specified: determined by the compiler.]),
   [case "$enableval" in
      no|off)
         # disable the default enabling:
         AC_DEFINE([PNG_INTEL_SSE_OPT], [0],
                   [Disable Intel SSE optimizations])
         # Prevent inclusion of the assembler files below:
         enable_intel_sse=no;;
      yes|on)
         AC_DEFINE([PNG_INTEL_SSE_OPT], [1],
                   [Enable Intel SSE optimizations]);;
      *)
         AC_MSG_ERROR([--enable-intel-sse=${enable_intel_sse}: invalid value])
   esac])

# Add Intel specific files to all builds where the host_cpu is Intel ('x86*')
# or where Intel optimizations were explicitly requested (this allows a
# fallback if a future host CPU does not match 'x86*')
AM_CONDITIONAL([PNG_INTEL_SSE],
   [test "$enable_intel_sse" != 'no' &&
    case "$host_cpu" in
      i?86|x86_64) :;;
      *)    test "$enable_intel_sse" != '';;
    esac])
-----------------cut----------------

3. Insert the following lines above the copyright line near the top of
Makefile.am:

-----------------cut----------------
# Copyright (c) 2016 Google, Inc.
# Written by Mike Klein and Matt Sarett
# Derived from the ARM supporting code in libpng/configure.ac, which was
-----------------cut----------------

4. Add the following code to Makefile.am under HOST SPECIFIC OPTIONS
directly beneath the "if PNG_ARM_NEON ... endif" statement:

-----------------cut----------------
if PNG_INTEL_SSE
libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_SOURCES += contrib/intel/intel_init.c\
    contrib/intel/filter_sse2_intrinsics.c
endif
-----------------cut----------------