contrib: png: update to 1.6.39

On 32-bit ARM, the cmake default is now -DPNG_ARM_NEON=off so we do runtime
detection for platforms where it's not already hard enabled.

contrib/src/png/SHA512SUMS

-4e450636062fcc75ecc65715e0b23ddc1097b73b4c95ffd31bef627144c576f58660b2130105f5f5781212cf54f00c7b6dd3facefd7e9de70c76b981d499f81e  libpng-1.6.38.tar.xz
+ce233bb58d70b391b7dc858a132a8f9f66a9e30f5b387426317fdd5e76e8395e00ec36324e5e2caec8fda49a3f2b13fe27594a3466fdf2020ec7520e79a608c9  libpng-1.6.39.tar.xz

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.6.38
+PNG_VERSION := 1.6.39
 PNG_URL := $(SF)/libpng/libpng16/$(PNG_VERSION)/libpng-$(PNG_VERSION).tar.xz
 
 PKGS += png
@@ -36,6 +36,13 @@ PNG_CONF += -DPNG_ARM_NEON=on
 else ifdef HAVE_WIN32
 # No runtime detection needed
 PNG_CONF += -DPNG_ARM_NEON=on
+else ifdef HAVE_ANDROID
+# libpng disallows "check" for ARM here as it would be redundant/"unproductive",
+# see https://github.com/glennrp/libpng/commit/b8ca9108acddfb9fb5d886f5e8a072ebaf436dbb
+PNG_CONF += -DPNG_ARM_NEON=on
+else
+# Otherwise do runtime detection
+PNG_CONF += -DPNG_ARM_NEON=check
 endif
 endif