cmake protobuf_generate(GENERATE_EXTENSIONS) examples

protocolbuffers/protobuf cmake/tests.cmake :202 

protobuf_generate(
  PROTOS ${proto_file}
  LANGUAGE ${generator}
  GENERATE_EXTENSIONS .${generator}.h .${generator}.c
  OUT_VAR pb_generated_files
  IMPORT_DIRS ${protobuf_SOURCE_DIR}/src
  IMPORT_DIRS ${protobuf_SOURCE_DIR}
  PLUGIN protoc-gen-${generator}=$<TARGET_FILE:protobuf::protoc-gen-${generator}>
  DEPENDENCIES $<TARGET_FILE:protobuf::protoc-gen-${generator}>
)