cmake: Fix the build on Windows.

src/threads/CMakeLists.txt

@@ -53,11 +53,12 @@ include_directories(
   ${LIBSSH_THREADS_PRIVATE_INCLUDE_DIRS}
 )
 
-add_library(${LIBSSH_THREADS_SHARED_LIBRARY} SHARED ${libssh_threads_SRCS})
+if (libssh_threads_SRCS)
+    add_library(${LIBSSH_THREADS_SHARED_LIBRARY} SHARED ${libssh_threads_SRCS})
 
-target_link_libraries(${LIBSSH_THREADS_SHARED_LIBRARY} ${LIBSSH_THREADS_LINK_LIBRARIES})
+    target_link_libraries(${LIBSSH_THREADS_SHARED_LIBRARY} ${LIBSSH_THREADS_LINK_LIBRARIES})
 
-set_target_properties(
+    set_target_properties(
       ${LIBSSH_THREADS_SHARED_LIBRARY}
         PROPERTIES
           VERSION
@@ -68,22 +69,22 @@ set_target_properties(
             ssh_threads
           DEFINE_SYMBOL
             LIBSSH_EXPORTS
-)
+    )
 
-if (WITH_VISIBILITY_HIDDEN)
+    if (WITH_VISIBILITY_HIDDEN)
       set_target_properties(${LIBSSH_THREADS_SHARED_LIBRARY} PROPERTIES COMPILE_FLAGS "-fvisibility=hidden")
-endif (WITH_VISIBILITY_HIDDEN)
+    endif (WITH_VISIBILITY_HIDDEN)
 
-install(
+    install(
       TARGETS
         ${LIBSSH_THREADS_SHARED_LIBRARY}
       RUNTIME DESTINATION ${BIN_INSTALL_DIR}
       LIBRARY DESTINATION ${LIB_INSTALL_DIR}
       ARCHIVE DESTINATION ${LIB_INSTALL_DIR}
       COMPONENT libraries
-)
+    )
 
-if (WITH_STATIC_LIB)
+    if (WITH_STATIC_LIB)
       add_library(${LIBSSH_THREADS_STATIC_LIBRARY} STATIC ${libssh_threads_SRCS})
 
       if (MSVC)
@@ -122,4 +123,5 @@ if (WITH_STATIC_LIB)
         COMPONENT
           libraries
       )
-endif (WITH_STATIC_LIB)
+    endif (WITH_STATIC_LIB)
+endif (libssh_threads_SRCS)