community/opensmtpd-extras: fix build with automake 1.17

Fix：https://build.alpinelinux.org/buildlogs/build-edge-loongarch64/community/opensmtpd-extras/opensmtpd-extras-6.7.1-r1.log

error：

./bootstrap: you need automake version 1.13 or later

conftest.c:49:2: error: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
   49 |  exit(0);
      |  ^~~~