Problem: --version-script not used on OpenBSD

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.

Makefile.am

@@ -294,20 +294,17 @@ else
 if ON_LINUX
 src_libzmq_la_LDFLAGS = \
 	-version-info @LTVER@ \
-	@LIBZMQ_EXTRA_LDFLAGS@ \
-	-Wl,--version-script=$(srcdir)/src/libzmq.vers
+	@LIBZMQ_EXTRA_LDFLAGS@
 else
 if ON_GNU
 src_libzmq_la_LDFLAGS = \
 	-version-info @LTVER@ \
-	@LIBZMQ_EXTRA_LDFLAGS@ \
-	-Wl,--version-script=$(srcdir)/src/libzmq.vers
+	@LIBZMQ_EXTRA_LDFLAGS@
 else
 if ON_DEBIAN_KFREEBSD
 src_libzmq_la_LDFLAGS = \
 	-version-info @LTVER@ \
-	@LIBZMQ_EXTRA_LDFLAGS@ \
-	-Wl,--version-script=$(srcdir)/src/libzmq.vers
+	@LIBZMQ_EXTRA_LDFLAGS@
 else
 src_libzmq_la_LDFLAGS = \
 	-version-info @LTVER@ \
@@ -320,6 +317,8 @@ endif
 endif
 endif
 
+src_libzmq_la_LDFLAGS += -export-symbols-regex '^zmq_.*'
+
 src_libzmq_la_CPPFLAGS = $(CODE_COVERAGE_CPPFLAGS) $(LIBUNWIND_CFLAGS)
 src_libzmq_la_CFLAGS = $(CODE_COVERAGE_CFLAGS) $(LIBUNWIND_CFLAGS)
 src_libzmq_la_CXXFLAGS = @LIBZMQ_EXTRA_CXXFLAGS@ $(CODE_COVERAGE_CXXFLAGS) \
@@ -1047,7 +1046,6 @@ EXTRA_DIST = \
 	version.sh	\
 	src/libzmq.pc.cmake.in \
 	ci_build.sh \
-	src/libzmq.vers \
 	src/version.rc.in \
 	tests/CMakeLists.txt \
 	unittests/CMakeLists.txt \

src/libzmq.vers

-{
-  global: zmq_*;
-  local: *;
-};