Fix Raspberry Pi build by making PNG not try to use Neon (by autodetect).

This involves patching to override the png neon option. In the future
it might be worth enabling PNG optimization.

PiperOrigin-RevId: 195846513

Author: aselle

tensorflow/workspace.bzl

@@ -228,6 +228,7 @@ def tf_workspace(path_prefix="", tf_repo_name=""):
       sha256 = "e45ce5f68b1d80e2cb9a2b601605b374bdf51e1798ef1c2c2bd62131dfcf9eef",
       strip_prefix = "libpng-1.6.34",
       build_file = clean_dep("//third_party:png.BUILD"),
+      patch_file = clean_dep("//third_party:png_fix_rpi.patch"),
   )
 
   tf_http_archive(

third_party/png_fix_rpi.patch

@@ -0,0 +1,16 @@
+diff -r -u /tmp/libpng-1.6.34/scripts/pnglibconf.h.prebuilt ./scripts/pnglibconf.h.prebuilt
+--- /tmp/libpng-1.6.34/scripts/pnglibconf.h.prebuilt	2017-09-29 01:42:33.000000000 -0700
++++ ./scripts/pnglibconf.h.prebuilt	2018-05-01 09:51:24.719318242 -0700
+@@ -20,6 +20,12 @@
+ #define PNG_ALIGNED_MEMORY_SUPPORTED
+ /*#undef PNG_ARM_NEON_API_SUPPORTED*/
+ /*#undef PNG_ARM_NEON_CHECK_SUPPORTED*/
++
++/* Workaround not having a great build file by forcing
++ * png filter optimization to be disabled on arm */
++#define PNG_ARM_NEON_OPT 0
++
++
+ /*#undef PNG_POWERPC_VSX_API_SUPPORTED*/
+ /*#undef PNG_POWERPC_VSX_CHECK_SUPPORTED*/
+ #define PNG_BENIGN_ERRORS_SUPPORTED