do not wipe out MacPorts values of CFLAGS just to set -O3

--- configure.ac.orig	2019-03-10 08:35:26.000000000 -0700
+++ configure.ac	2021-04-11 09:06:12.000000000 -0700
@@ -60,7 +60,7 @@
                [enable_native=yes]
 )
 
-AX_CHECK_COMPILE_FLAG([-O3], [CFLAGS=-O3])
+dnl AX_CHECK_COMPILE_FLAG([-O3], [CFLAGS=-O3])
 dnl Not all architectures support -march=native
 if test $enable_native == "yes"; then
   AX_CHECK_COMPILE_FLAG([-march=native], [], [enable_native=no])