It's necessary to set this var to "yes" _after_ AC_DEFINE and AC_SUBST
in order for a later `test` to pass so that `check_for_ca_bundle=1` ends
up being set. This is in turn required for the default CA certificate
bundle to be set when building w/ rustls & pkg-config.
Reported-by: Matt Jolly
Fixes#13248Closes#13251