Libsodium is now required if and only if --with-libsodium is used

This patch is proposed for https://github.com/zeromq/libzmq/issues/912.  If a user specifies that they want libzmq built with libsodium, and the requirement cannot be satisfied, we should error at the user so they can either A) install libsodium to a reasonable location or B) relax their requirement for libsodium support.

Previously a warning was issued in this case, causing users who wanted libsodium support not to get it, which may be discovered at a much later time when their programs didn't work as expected.

I release this patch under the LGPL v3 or any later version.
Merge pull request #914 from drewcrawford/issue_912

Libsodium is now required if and only if --with-libsodium is used
Add the tag name to the zmq_pollitem_t struct to allow easier forward declaration
Merge pull request #920 from mbarbisan/master

Add the tag name to the zmq_pollitem_t struct to allow easier forward de...
Allow TCP addresses to be re-resolved whenever there is a reconnection attempt
Merge pull request #921 from mbarbisan/master

Allow TCP addresses to be re-resolved whenever there is a reconnection a...
Fixed potential SEGFAULT.
Merge remote-tracking branch 'upstream/master'
Merge pull request #922 from horus42/master

Fixed potential SEGFAULT in socket_base.cpp
Revert "Fixed potential SEGFAULT."

This reverts commit 79b81f48402c1276d4cfd3bf0cbfb4084952b36b.
Was causing:

Assertion failed: destroyed (socket_base.cpp:154)
/bin/bash: line 5: 31344 Aborted                 ${dir}$tst
FAIL: test_many_sockets

On TravisCI.
Merge pull request #923 from hintjens/master

Revert "Fixed potential SEGFAULT."