Build error with gcc on OpenBSD

[jcourreges]

Issue description

zeromq fails to build using gcc on OpenBSD. This is not visible using the clang compiler, like done by default on mainstream architectures like amd64, i386, arm and arm64.

Environment

• libzmq version (commit hash if unreleased):
  0MQ version 4.3.1, commit f615b8f

• OS:
  OpenBSD 6.4 (-current) amd64

Minimal test code / Steps to reproduce the issue

./autogen.sh && CC=egcc CXX=eg++ ./configure --disable-Werror --with-libsodium --prefix='/usr/local' --sysconfdir='/etc' --mandir='/usr/local/man' --infodir='/usr/local/info' --localstatedir='/var' --disable-silent-rules && gmake LIBTOOL=/usr/bin/libtool

What's the actual result? (include assertion message & call stack if applicable)

Short error message:
eg++: error: unrecognized command line option '-Wl'

Full log not included because github ui...
External log available at http://build-failures.rhaalovely.net//sparc64/2018-11-15/net/zeromq.log