Specifically when cross-compiling, one can be willing to force these
variable values using the environment of a config.cache file. This
commit makes this possible.

The affected variables are:

* libzmq_cv_sock_cloexec
* libzmq_cv_so_keepalive
* libzmq_cv_tcp_keepcnt
* libzmq_cv_tcp_keepidle
* libzmq_cv_tcp_keepintvl
* libzmq_cv_tcp_keepalive
Signed-off-by: Lionel Orry <lionel.orry@gmail.com>