CMake: Enable projects to set the C++ version (#10464)

This change enables projects that consume protobuf via
`FetchContent_MakeAvailable()` to set the C++ version to be used. This is
necessary, as linking code compiled for different C++ standards is asking for
trouble (and will simply not work in some cases).

Check that any version that might be set in `CMAKE_CXX_STANDARD` is new
enough (C++14 or later). On Cygwin, check if any `-std=gnu++XX` has
already been set. In all cases, default to C++14.

CMakeLists.txt

@@ -32,11 +32,23 @@ if(protobuf_DEPRECATED_CMAKE_SUBDIRECTORY_USAGE)
   get_filename_component(protobuf_SOURCE_DIR ${protobuf_SOURCE_DIR} DIRECTORY)
 endif()
 
-# Add c++14 flags
-if (CYGWIN)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++14")
-else()
+# Add C++14 flags
+if(NOT CMAKE_CXX_STANDARD)
   set(CMAKE_CXX_STANDARD 14)
+endif()
+if(CYGWIN)
+  string(REGEX_MATCH "-std=gnu\\+\\+([0-9]+)" _protobuf_CXX_STD "${CMAKE_CXX_FLAGS}")
+endif()
+if(NOT _protobuf_CXX_STD)
+  set(_protobuf_CXX_STD "${CMAKE_CXX_STANDARD}")
+endif()
+if(_protobuf_CXX_STD LESS "14")
+  message(FATAL_ERROR "Protocol Buffers requires at least C++14, but is configured for C++${_protobuf_CXX_STD}")
+endif()
+if(CYGWIN)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++${_protobuf_CXX_STD}")
+else()
+  set(CMAKE_CXX_STANDARD ${_protobuf_CXX_STD})
   set(CMAKE_CXX_STANDARD_REQUIRED ON)
   set(CMAKE_CXX_EXTENSIONS OFF)
 endif()