Merge pull request #2235 from bluca/cmake_soversion

Problem: CMake build uses library version as the ABI version

CMakeLists.txt

@@ -733,11 +733,12 @@ if (MSVC)
     OUTPUT_NAME "libzmq-static")
 else ()
     add_library (libzmq SHARED ${sources} ${public_headers} ${html-docs} ${readme-docs} ${zmq-pkgconfig})
+    # NOTE: the SOVERSION MUST be the same as the one generated by libtool!
     set_target_properties (libzmq PROPERTIES
                           COMPILE_DEFINITIONS "DLL_EXPORT"
                           PUBLIC_HEADER "${public_headers}"
                           VERSION ${ZMQ_VERSION}
-                          SOVERSION "${ZMQ_VERSION_MAJOR}.${ZMQ_VERSION_MINOR}.0"
+                          SOVERSION "5.1.0"
                           OUTPUT_NAME "libzmq"
                           PREFIX "")
     if (ZMQ_BUILD_FRAMEWORK)