gcf2: fix loading at runtime

gcf2: fix loading at runtime

We need to waitpid synchronously on pkg-config to use $?.

When loading Gcf2 inside the event loop, implicit dwaitpid
done by PublicInbox::ProcessPipe would not call waitpid in
time to zero $?.  This was causing one of my -httpd to
occasionally fall back to git(1) instead of using Gcf2.

This was noted in:
Link: https://public-inbox.org/meta/20210914085322.25517-1-e@80x24.org/