Solution: go back to using -Wl,--version-script.
Use ax_check_vscript.m4 from the autoconf-archive to detect support on
multiple platforms (eg Solaris ld(1) -M).
libtool -export-symbols-regexp used ld(1) --retain-symbols-file under
the hood, the latter lets some C++ weak symbols make their way into the
dynamic symbols table, along with the zmq_* interface. The reason for
such behavior is unknown to me.
Solution: switch to libtool symbol visibility support, which leads to
more portable constructs in the autotools glue. As noted in the libtool
documentation "This option has no effect on some platforms."
After this commit, global symbols intended to be kept private will
disappear on platforms that were previously not handled
using -Wl,--version-script.
