src/test/examples/testlibpq2.c where the HAVE_SYS_SELECT_H is defined.

Hi,
playing around with $[0] testlibpq2.c example. I wondered where
HAVE_SYS_SELECT_H is defined?

I searched on the internet, founded that people also asked the same
question in $[1].

In my machine, I do have <sys/select.h>.
system version: Ubuntu 22.04.1 LTS
gcc version: gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
gcc compile command: gcc pg_testlibpq2.c -I/home/jian/postgres/pg16/include
\
-L/home/jian/postgres/pg16/lib -lpq

[0]https://www.postgresql.org/docs/current/libpq-example.html
[1]
https://stackoverflow.com/questions/37876850/in-compilation-time-how-to-find-the-macro-is-defined-in-which-header-file