Added : ability to select memory access mode externally (on command l…

…ine for example)

xxhash.c

@@ -48,11 +48,13 @@ You can contact the author at :
  * See http://stackoverflow.com/a/32095106/646947 for details.
  * Prefer these methods in priority order (0 > 1 > 2)
  */
-#if defined(__GNUC__) && ( defined(__ARM_ARCH_6__) || defined(__ARM_ARCH_6J__) || defined(__ARM_ARCH_6K__) || defined(__ARM_ARCH_6Z__) || defined(__ARM_ARCH_6ZK__) || defined(__ARM_ARCH_6T2__) )
-#  define XXH_FORCE_MEMORY_ACCESS 2
-#elif defined(__INTEL_COMPILER) || \
+#ifndef XXH_FORCE_MEMORY_ACCESS   /* can be defined externally, on command line for example */
+#  if defined(__GNUC__) && ( defined(__ARM_ARCH_6__) || defined(__ARM_ARCH_6J__) || defined(__ARM_ARCH_6K__) || defined(__ARM_ARCH_6Z__) || defined(__ARM_ARCH_6ZK__) || defined(__ARM_ARCH_6T2__) )
+#    define XXH_FORCE_MEMORY_ACCESS 2
+#  elif defined(__INTEL_COMPILER) || \
   (defined(__GNUC__) && ( defined(__ARM_ARCH_7__) || defined(__ARM_ARCH_7A__) || defined(__ARM_ARCH_7R__) || defined(__ARM_ARCH_7M__) || defined(__ARM_ARCH_7S__) ))
-#  define XXH_FORCE_MEMORY_ACCESS 1
+#    define XXH_FORCE_MEMORY_ACCESS 1
+#  endif
 #endif
 
 /* XXH_ACCEPT_NULL_INPUT_POINTER :