[core] define static_assert for uClibc (fixes #3127)

(thx aeolio)

define static_assert in algo_xxhash.h for uClibc

x-ref:
  "src/algo_xxhash.h: build failure with toolchain using uClibc"
  https://redmine.lighttpd.net/issues/3127

src/algo_xxhash.h

@@ -1563,6 +1563,9 @@ static void* XXH_memcpy(void* dest, const void* src, size_t size)
 #ifndef XXH_STATIC_ASSERT
 #  if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)    /* C11 */
 #    include <assert.h>
+#    ifndef static_assert /* uClibc does not define static_assert */
+#      define static_assert _Static_assert
+#    endif
 #    define XXH_STATIC_ASSERT_WITH_MESSAGE(c,m) do { static_assert((c),m); } while(0)
 #  elif defined(__cplusplus) && (__cplusplus >= 201103L)            /* C++11 */
 #    define XXH_STATIC_ASSERT_WITH_MESSAGE(c,m) do { static_assert((c),m); } while(0)