diff --git a/pngpriv.h b/pngpriv.h
index 5980a3fc3..a767f7753 100644
--- a/pngpriv.h
+++ b/pngpriv.h
@@ -118,8 +118,12 @@
     * to compile with an appropriate #error if ALIGNED_MEMORY has been turned
     * off.
     *
-    * Note that gcc-4.9 defines __ARM_NEON instead of __ARM_NEON__, so we
-    * check both variants.
+    * Note that gcc-4.9 defines __ARM_NEON instead of the deprecated
+    * __ARM_NEON__, so we check both variants.
+    *
+    * To disable ARM_NEON optimizations entirely, and skip compiling the
+    * associated assembler code, pass --enable-arm-neon=no to configure
+    * or put -DPNG_ARM_NEON_OPT=0 in CPPFLAGS.
     */
 #  if (defined(__ARM_NEON__) || defined(__ARM_NEON)) && \
    defined(PNG_ALIGNED_MEMORY_SUPPORTED)