--- a/m4/ax_ext.m4
+++ b/m4/ax_ext.m4
@@ -164,6 +164,7 @@ AC_DEFUN([AX_EXT],
         fi
       fi
 
+      AC_CHECK_SIZEOF([long])
       if test "$ac_cv_sizeof_long" -eq 8; then
           SIMD_FLAGS="$SIMD_FLAGS -DARCH_64"
       fi