[libpng17] Make symbol prefixing work with the ARM neon optimizations. Also

allow pngpriv.h to be included for preprocessor definitions only, so
it can be used in non-C/C++ files.

pngmem.c

@@ -154,10 +154,12 @@ png_realloc_array,(png_structrp png_ptr, png_const_voidp old_array,
       }
    }
 
+#ifdef PNG_READ_SUPPORTED
    /* The potential overflow case.  Set the cache counter so libpng will
     * not make any more attempts
     */
    png_ptr->user_chunk_cache_max = 2;
+#endif
 
    return NULL; /* error */
 }