CMake fixes (#9595)

* Support older versions of CMake.

VERSION_GREATER_EQUAL is supported by CMake >=3.7:
https://cmake.org/cmake/help/latest/command/if.html#version-greater-equal

target_link_options() is supported by CMake >=3.13:
https://cmake.org/cmake/help/latest/command/target_link_options.html

* Use PROTOBUF_USE_DLLS and LIBPROTOC_EXPORTS for libprotoc.so shared library

Similar change for libprotobuf.so and libprotobuf-lite.so was made
in commit 5a0887fc65.

* Make value of PROTOC_EXPORT dependent on LIBPROTOC_EXPORTS instead of LIBPROTOBUF_EXPORTS

This was probably intention, and LIBPROTOC_EXPORTS is also used above
in _MSC_VER branch.

cmake/CMakeLists.txt

@@ -135,7 +135,7 @@ file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/cmaketest.map
     *;
 };")
 # CheckLinkerFlag module available in CMake >=3.18.
-if(${CMAKE_VERSION} VERSION_GREATER_EQUAL 3.18)
+if(${CMAKE_VERSION} VERSION_GREATER 3.18 OR ${CMAKE_VERSION} VERSION_EQUAL 3.18)
   include(CheckLinkerFlag)
   check_linker_flag(CXX -Wl,--version-script=${CMAKE_CURRENT_BINARY_DIR}/cmaketest.map protobuf_HAVE_LD_VERSION_SCRIPT)
 else()
@@ -206,7 +206,7 @@ else (protobuf_BUILD_SHARED_LIBS)
   # Prior to CMake 3.15, the MSVC runtime library was pushed into the same flags
   # making programmatic control difficult.  Prefer the functionality in newer
   # CMake versions when available.
-  if(${CMAKE_VERSION} VERSION_GREATER_EQUAL 3.15)
+  if(${CMAKE_VERSION} VERSION_GREATER 3.15 OR ${CMAKE_VERSION} VERSION_EQUAL 3.15)
     if (protobuf_MSVC_STATIC_RUNTIME)
         set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded$<$<CONFIG:Debug>:Debug>)
     else()

cmake/libprotobuf-lite.cmake

@@ -94,16 +94,20 @@ endif()
 add_library(libprotobuf-lite ${protobuf_SHARED_OR_STATIC}
   ${libprotobuf_lite_files} ${libprotobuf_lite_includes} ${libprotobuf_lite_rc_files})
 if(protobuf_HAVE_LD_VERSION_SCRIPT)
-  target_link_options(libprotobuf-lite PRIVATE -Wl,--version-script=${protobuf_source_dir}/src/libprotobuf-lite.map)
+  if(${CMAKE_VERSION} VERSION_GREATER 3.13 OR ${CMAKE_VERSION} VERSION_EQUAL 3.13)
+    target_link_options(libprotobuf-lite PRIVATE -Wl,--version-script=${protobuf_source_dir}/src/libprotobuf-lite.map)
+  elseif(protobuf_BUILD_SHARED_LIBS)
+    target_link_libraries(libprotobuf-lite PRIVATE -Wl,--version-script=${protobuf_source_dir}/src/libprotobuf-lite.map)
+  endif()
   set_target_properties(libprotobuf-lite PROPERTIES
     LINK_DEPENDS ${protobuf_source_dir}/src/libprotobuf-lite.map)
 endif()
-target_link_libraries(libprotobuf-lite ${CMAKE_THREAD_LIBS_INIT})
+target_link_libraries(libprotobuf-lite PRIVATE ${CMAKE_THREAD_LIBS_INIT})
 if(protobuf_LINK_LIBATOMIC)
-  target_link_libraries(libprotobuf-lite atomic)
+  target_link_libraries(libprotobuf-lite PRIVATE atomic)
 endif()
 if(${CMAKE_SYSTEM_NAME} STREQUAL "Android")
-	target_link_libraries(libprotobuf-lite log)
+  target_link_libraries(libprotobuf-lite PRIVATE log)
 endif()
 target_include_directories(libprotobuf-lite PUBLIC ${protobuf_source_dir}/src)
 if(protobuf_BUILD_SHARED_LIBS)

cmake/libprotobuf.cmake

@@ -108,19 +108,23 @@ endif()
 add_library(libprotobuf ${protobuf_SHARED_OR_STATIC}
   ${libprotobuf_lite_files} ${libprotobuf_files} ${libprotobuf_includes} ${libprotobuf_rc_files})
 if(protobuf_HAVE_LD_VERSION_SCRIPT)
-  target_link_options(libprotobuf PRIVATE -Wl,--version-script=${protobuf_source_dir}/src/libprotobuf.map)
+  if(${CMAKE_VERSION} VERSION_GREATER 3.13 OR ${CMAKE_VERSION} VERSION_EQUAL 3.13)
+    target_link_options(libprotobuf PRIVATE -Wl,--version-script=${protobuf_source_dir}/src/libprotobuf.map)
+  elseif(protobuf_BUILD_SHARED_LIBS)
+    target_link_libraries(libprotobuf PRIVATE -Wl,--version-script=${protobuf_source_dir}/src/libprotobuf.map)
+  endif()
   set_target_properties(libprotobuf PROPERTIES
     LINK_DEPENDS ${protobuf_source_dir}/src/libprotobuf.map)
 endif()
-target_link_libraries(libprotobuf ${CMAKE_THREAD_LIBS_INIT})
+target_link_libraries(libprotobuf PRIVATE ${CMAKE_THREAD_LIBS_INIT})
 if(protobuf_WITH_ZLIB)
-  target_link_libraries(libprotobuf ${ZLIB_LIBRARIES})
+  target_link_libraries(libprotobuf PRIVATE ${ZLIB_LIBRARIES})
 endif()
 if(protobuf_LINK_LIBATOMIC)
-  target_link_libraries(libprotobuf atomic)
+  target_link_libraries(libprotobuf PRIVATE atomic)
 endif()
 if(${CMAKE_SYSTEM_NAME} STREQUAL "Android")
-	target_link_libraries(libprotobuf log)
+  target_link_libraries(libprotobuf PRIVATE log)
 endif()
 target_include_directories(libprotobuf PUBLIC ${protobuf_source_dir}/src)
 if(protobuf_BUILD_SHARED_LIBS)

cmake/libprotoc.cmake

@@ -132,12 +132,16 @@ endif()
 add_library(libprotoc ${protobuf_SHARED_OR_STATIC}
   ${libprotoc_files} ${libprotoc_headers} ${libprotoc_rc_files})
 if(protobuf_HAVE_LD_VERSION_SCRIPT)
-  target_link_options(libprotoc PRIVATE -Wl,--version-script=${protobuf_source_dir}/src/libprotoc.map)
+  if(${CMAKE_VERSION} VERSION_GREATER 3.13 OR ${CMAKE_VERSION} VERSION_EQUAL 3.13)
+    target_link_options(libprotoc PRIVATE -Wl,--version-script=${protobuf_source_dir}/src/libprotoc.map)
+  elseif(protobuf_BUILD_SHARED_LIBS)
+    target_link_libraries(libprotoc PRIVATE -Wl,--version-script=${protobuf_source_dir}/src/libprotoc.map)
+  endif()
   set_target_properties(libprotoc PROPERTIES
     LINK_DEPENDS ${protobuf_source_dir}/src/libprotoc.map)
 endif()
-target_link_libraries(libprotoc libprotobuf)
-if(MSVC AND protobuf_BUILD_SHARED_LIBS)
+target_link_libraries(libprotoc PRIVATE libprotobuf)
+if(protobuf_BUILD_SHARED_LIBS)
   target_compile_definitions(libprotoc
     PUBLIC  PROTOBUF_USE_DLLS
     PRIVATE LIBPROTOC_EXPORTS)

src/google/protobuf/port_def.inc

@@ -454,7 +454,7 @@
 # else
 #  define PROTOC_EXPORT __declspec(dllimport)
 # endif  // defined(LIBPROTOC_EXPORTS)
-#elif defined(PROTOBUF_USE_DLLS) && defined(LIBPROTOBUF_EXPORTS)
+#elif defined(PROTOBUF_USE_DLLS) && defined(LIBPROTOC_EXPORTS)
 # define PROTOC_EXPORT __attribute__((visibility("default")))
 #else
 # define PROTOC_EXPORT