add missing OPENSSL define to CMakeLists.txt

Signed-off-by: Ryan Pendleton <me@ryanp.me>

src/CMakeLists.txt

@@ -86,7 +86,8 @@ IF (PAHO_WITH_SSL)
     SET_TARGET_PROPERTIES(
         paho-mqtt3cs paho-mqtt3as PROPERTIES
         VERSION ${CLIENT_VERSION}
-        SOVERSION ${PAHO_VERSION_MAJOR})
+        SOVERSION ${PAHO_VERSION_MAJOR}
+        COMPILE_DEFINITIONS "OPENSSL=1")
     INSTALL(TARGETS paho-mqtt3cs
         RUNTIME DESTINATION bin
         LIBRARY DESTINATION lib)