return values of socket functions are ssize_t (not int) in compliance with POSIX

deprecated API (zmq_msg_recv/zmq_msg_send) still returns int
Merge pull request #1183 from mschneider/size_fix

ZMQ API diverges from POSIX
Revert "ZMQ API diverges from POSIX"
Merge pull request #1222 from zeromq/revert-1183-size_fix

Revert "ZMQ API diverges from POSIX"
Improve build script error checking.

This commit adds a check to make sure pkg-config is present on the
computer building libzmq.
This particular line was stolen from the debian mailing list.
See https://lists.debian.org/debian-dpkg/2011/04/msg00054.html
Merge pull request #1223 from xaqq/build-script

Improve build script error checking.
Fix issue #1224
Merge pull request #1225 from hurtonm/master

Fix issue #1224
Add --with-libgssapi_krb5 to match libsodium and pgm.
Add support for --with/--without libgssapi_krb5
Merge pull request #1226 from pmienk/master

Standardize optional libgssapi_krb5 dependency