[libpng16] Changed NEON implementation selection mechanism to allow assembler

or intrinsics to be turned on during the build by defining
PNG_ARM_NEON_IMPLEMENTATION to the correct value (2 or 1).  The
default is "undefined" and the build type is selected in pngpriv.h.

pngpriv.h

@@ -126,36 +126,49 @@
     * callbacks to do this.
     */
 #  define PNG_FILTER_OPTIMIZATIONS png_init_filter_functions_neon
-#endif
 
-#ifndef PNG_USE_ARM_NEON_ASM
    /* By default the 'intrinsics' code in arm/filter_neon_intrinsics.c is used
     * if possible - if __ARM_NEON__ is set and the compiler version is not known
-    * to be broken.  ASM code can be forced by -DPNG_USE_ARM_NEON_ASM on the
-    * command line.
+    * to be broken.  This is control by PNG_ARM_NEON_IMPLEMENTATION which can
+    * be:
+    *
+    *    1  The intrinsics code (the default with __ARM_NEON__)
+    *    2  The hand coded assembler (the default without __ARM_NEON__)
+    *
+    * It is possible to set PNG_ARM_NEON_IMPLEMENTATION in CPPFLAGS, however
+    * this is *NOT* supported and may cease to work even after a minor revision
+    * to libpng.  It *is* valid to do this for testing purposes, e.g. speed
+    * testing or a new compiler, but the results should be communicated to the
+    * libpng implementation list for incorporation in the next minor release.
     */
-#  ifndef __ARM_NEON__
-      /* The 'intrinsics' code simply won't compile without this -mfpu=neon: */
-#     define PNG_USE_ARM_NEON_ASM
-#  endif
-#endif
+#  ifndef PNG_ARM_NEON_IMPLEMENTATION
+#     ifdef __ARM_NEON__
+#        if defined(__clang__)
+            /* At present it is unknown by the libpng developers which versions
+             * of clang support the intrinsics, however some or perhaps all
+             * versions do not work with the assembler so this may be
+             * irrelevant, so just use the default (do nothing here.)
+             */
+#        elif defined(__GNUC__)
+            /* GCC 4.5.4 NEON support is known to be broken.  4.6.3 is known to
+             * work, so if this *is* GCC, or G++, look for a version >4.5
+             */
+#           if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 6)
+#              define PNG_ARM_NEON_IMPLEMENTATION 2
+#           endif /* no GNUC support */
+#        endif /* __GNUC__ */
+#     else /* !__ARM_NEON__ */
+         /* The 'intrinsics' code simply won't compile without this -mfpu=neon:
+          */
+#        define PNG_ARM_NEON_IMPLEMENTATION 2
+#     endif /* !__ARM_NEON */
+#  endif /* !defined PNG_ARM_NEON_IMPLEMENTATION */
 
-   /* Put compiler specific checks below: */
-#ifndef PNG_USE_ARM_NEON_ASM
-#  if defined(__clang__)
-      /* At present it is unknown by the libpng developers which versions of
-       * clang support the intrinsics, however some or perhaps all versions
-       * do not work with the assembler so this may be irrelevant.
-       */
-#  elif defined(__GNUC__)
-      /* GCC 4.5.4 NEON support is known to be broken.  4.6.3 is known to work,
-       * so if this *is* GCC, or G++, look for a version >4.5
-       */
-#     if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 6)
-#        define PNG_USE_ARM_NEON_ASM
-#     endif
-#  endif /* __GNUC__ */
-#endif
+#  ifndef PNG_ARM_NEON_IMPLEMENTATION
+      /* Use the intrinsics code by default. */
+#     define PNG_ARM_NEON_IMPLEMENTATION 1
+#  endif
+#endif /* PNG_ARM_NEON_OPT > 0 */
 
 /* Is this a build of a DLL where compilation of the object modules requires
  * different preprocessor settings to those required for a simple library?  If