Do not overwrite defaults configuration file

This file may contain customized configuration options
which should not be overwritten on install.

CMakeLists.txt

 install(TARGETS ${PROJECT_NAME} DESTINATION "sbin")
 install(FILES README.md main.cf.ex DESTINATION "share/doc/${PROJECT_NAME}")
 install(PROGRAMS ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.init DESTINATION "/etc/init.d" RENAME ${PROJECT_NAME})
-install(FILES ${PROJECT_NAME}.default DESTINATION "/etc/default" RENAME ${PROJECT_NAME})
 install(SCRIPT "${CMAKE_CURRENT_BINARY_DIR}/postinstall.cmake")
 

postinstall.cmake.in

 	COMMAND ln -sf ../init.d/@PROJECT_NAME@ rc6.d/K20@PROJECT_NAME@
 	WORKING_DIRECTORY "$ENV{DESTDIR}/etc"
 )
+if(NOT EXISTS "$ENV{DESTDIR}/etc/default/@PROJECT_NAME@")
+	file(INSTALL "@CMAKE_CURRENT_SOURCE_DIR@/@PROJECT_NAME@.default" DESTINATION "/etc/default" RENAME "@PROJECT_NAME@")
+else()
+	message(STATUS "Keeping old version: $ENV{DESTDIR}/etc/default/@PROJECT_NAME@")
+endif()
 set(SECRET_FILE "@PROJECT_NAME@.secret")
 set(DD "@DD@")
 set(BASE64 "@BASE64@")