Use NEON on ARM hardware via sse2neon.h

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

configure.ac

@@ -73,19 +73,52 @@ 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])
+   ])
+
+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],