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.