Merge pull request #4 from hintjens/master

Backported fixes from master
Add --with-libgssapi_krb5 to match libsodium and pgm.
Add support for --with/--without libgssapi_krb5
Pass --with/--without-libgssapi_krb5 to configure.
Correct libgssapi_krb5 packages.
Support both --enable-eventfd and --disable-eventfd options.
Merge pull request #5 from johntconklin/master

backport configure and rpmbuild fixes.
Unbind socket with real endpoint when binding by wild-card * address
Merge pull request #6 from lysyloren/master

Unbind socket with real endpoint when binding by wild-card * address
Problem: zmq_ctx_term has insane behavior by default

Solution: document this with a clear warning. It would be
nicer perhaps to change the default LINGER to e.g. a few
seconds. However this could break existing applications.