Use AC_CHECK_DECLS, suggestion by Thomas J.

configure.ac

@@ -30,16 +30,7 @@ AC_TYPE_UINT8_T
 
 # Checks for library functions.
 AC_CHECK_FUNCS([memset strstr])
-AC_MSG_CHECKING(for __builtin_clzll)
-AC_TRY_LINK([],[
-  __builtin_clzll(1);
-],[
-  AC_DEFINE(AEC_HAVE___BUILTIN_CLZLL, 1, \
-  [Define to 1 if you have the '__builtin__clzll' function.])
-  AC_MSG_RESULT(yes)
-],[
-  AC_MSG_RESULT(no)
-])
+AC_CHECK_DECLS(__builtin_clzll)
 
 AC_CONFIG_FILES([Makefile         \
                  src/Makefile     \

src/decode.c

@@ -277,7 +277,7 @@ static inline uint32_t direct_get_fs(struct aec_stream *strm)
     if (state->acc == 0)
         fill_acc(strm);
 
-#ifdef AEC_HAVE___BUILTIN_CLZLL
+#ifdef HAVE_DECL___BUILTIN_CLZLL
     fs = __builtin_clzll(state->acc) - (64 - state->bitp);
     state->bitp -= fs + 1;
 #else