objective-caml: fix clang; drop -fno-defer-pop.

Recent versions of clang fail with a hard error if -fno-defer-pop is
specified, and older versions warn. This patch fixes the OCaml
configure script to not pass this option as it hasn't been necessary
for decades.

See ocaml/opam#1242

Closes #27611.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>