Use NEON on ARM hardware via sse2neon.h

The autoconf changes were adapted from:
https://github.com/glennrp/libpng/blob/libpng16/configure.ac

windows/configure.ac

@@ -73,19 +73,126 @@ AS_IF([test "x$FOUND_SHUF" = xyes],  [AC_DEFINE([HAVE_SHUF], [1], [shuf availabl
 AS_IF([test "x$FOUND_GSHUF" = xyes],  [AC_DEFINE([HAVE_GSHUF], [1], [gshuf available])])
 
 # ------------------------------------------------------------------
-# Checks for SSE2 build
+# Architecture-specific options
 # ------------------------------------------------------------------
-AC_ARG_ENABLE([sse2],
-    AS_HELP_STRING(
-        [--disable-sse2],
-        [disable SSE2 optimization routines]
-        )
-    )
 
-AS_IF([test "x$enable_sse2" != "xno"], [
-    CFLAGS="-mfpmath=sse -msse2 -DUSE_SSE ${CFLAGS}"
+# allow enabling hardware optimization on any system:
+case "$host_cpu" in
+   arm*|aarch64*)
+     enable_arm_neon=yes
+     enable_intel_sse=no
+     AC_DEFINE([ARM_NEON], [1],
+       [Enable ARM_NEON optimizations])
+     ;;
+   i?86|x86_64)
+     enable_intel_sse=yes
+     enable_arm_neon=no
+     AC_DEFINE([INTEL_SSE], [1],
+       [Enable Intel SSE optimizations])
+     ;;
+esac
+
+AC_ARG_ENABLE([hardware-optimizations],
+   AS_HELP_STRING([[[--disable-hardware-optimizations]]],
+      [Disable hardware optimizations (Intel SSE2 / ARM NEON)]),
+   [
+     # disable hardware optimization on all systems:
+     enable_arm_neon=no
+     AC_DEFINE([ARM_NEON], [0],
+       [Disable ARM_NEON optimizations])
+     enable_intel_sse=no
+     AC_DEFINE([INTEL_SSE], [0],
+       [Disable INTEL_SSE optimizations])
+   ])
+
+# 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 "$enableval" in
+     no|off)
+        # disable the default enabling:
+        AC_DEFINE([INTEL_SSE], [0],
+                  [Disable Intel SSE optimizations])
+        # Prevent inclusion of the assembler files below:
+        enable_intel_sse=no;;
+     yes|on)
+        enable_intel_sse=yes
+        AC_DEFINE([INTEL_SSE], [1],
+                  [Enable Intel SSE optimizations]);;
+     *)
+        AC_MSG_ERROR([--enable-intel-sse=${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([INTEL_SSE],
+  [test "$enable_intel_sse" != 'no' &&
+   case "$host_cpu" in
+     i?86|x86_64) :;;
+     *)    test "$enable_intel_sse" != '';;
+   esac])
+
+# ARM
+# ===
+#
+# ARM NEON (SIMD) support.
+
+AC_ARG_ENABLE([arm-neon],
+  AS_HELP_STRING([[[--enable-arm-neon]]],
+     [Enable ARM NEON optimizations: =no/off, check, api, yes/on:]
+     [no/off: disable the optimizations; check: use internal checking code]
+     [(deprecated and poorly supported); api: disable by default, enable by]
+     [a call to png_set_option; yes/on: turn on unconditionally.]
+     [If not specified: determined by the compiler.]),
+  [case "$enableval" in
+     no|off)
+        # disable the default enabling on __ARM_NEON__ systems:
+        AC_DEFINE([ARM_NEON], [0],
+                  [Disable ARM Neon optimizations])
+        # Prevent inclusion of the assembler files below:
+        enable_arm_neon=no;;
+     yes|on)
+        enable_arm_neon=yes
+        AC_DEFINE([ARM_NEON], [1],
+                  [Enable ARM Neon optimizations]);;
+     *)
+        AC_MSG_ERROR([--enable-arm-neon=${enable_arm_neon}: invalid value])
+  esac])
+
+# Add ARM specific files to all builds where the host_cpu is arm ('arm*') or
+# where ARM optimizations were explicitly requested (this allows a fallback if a
+# future host CPU does not match 'arm*')
+
+AM_CONDITIONAL([ARM_NEON],
+  [test "$enable_arm_neon" != 'no' &&
+   case "$host_cpu" in
+     arm*|aarch64*) :;;
+     *)    test "$enable_arm_neon" != '';;
+   esac])
+
+SIMDFLAGS=""
+
+AS_IF([test "x$enable_intel_sse" != "xno"], [
+  SIMDFLAGS="-mfpmath=sse -msse2 -DINTEL_SSE"
 ])
 
+AS_IF([test "x$enable_arm_neon" != "xno"], [
+  SIMDFLAGS="-march=armv8-a+fp+simd+crypto+crc -DARM_NEON"
+])
+
+CFLAGS="${SIMDFLAGS} ${CFLAGS}"
+
+AC_SUBST([SIMDFLAGS], [$SIMDFLAGS])
+
 AC_CHECK_HEADER(cblas.h, [AX_CBLAS])
 
 AC_ARG_ENABLE([data-download],