Standardize optional libgssapi_krb5 dependency

Fix issue #1224

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

Revert "ZMQ API diverges from POSIX"

ZMQ API diverges from POSIX

Update travis script to configure with libsodium.

Support --with and --without options for libsodium and pgm

command line options.

Append to libzmq_la_CPPFLAGS and libzmq_la_LIBADD.

Remove build and runtime dependencies on e2fsprogs and e2fsprogs-devel.

Add support for POLLPRI flag.

This commit adds a ZMQ_POLLPRI flag that maps to poll()'s POLLPRI
flag.
This flags does nothing for OMQ sockets. It's only useful for raw
file descriptor (be it socket or file).
This flag does nothing if poll() is not the underlying polling
function. So it is Linux only.

Releasing 4.1.0 rc1 and moving development master to 4.2.0.

Fix builds/msvc/Makefile.am to include all properties files in the build

Fix some typos

I attempted to fix up the properties files manifests in the Makefile,
but neglected the end-of-line escape characters.  It was good enough
to pass whatever make dist on Linux did, but I had no Windows build
env to test in.  The broken change was in commit de4a4422.
Signed-off-by: Dan Mick <dan.mick@inktank.com>

Correct libsodium inclusion.

Simplify libsodium inclusion

Reduce automake recursion; simplify pgm dependency

PVS Studio small fix

Fix V815 Decreased performance. Consider replacing the expression 'peer_address = ""' with 'peer_address.clear()'. stream_engine.cpp 99