smarter version.c build that avoids full rebuilds.

src/CMakeLists.txt

@@ -171,7 +171,10 @@ set (SERVER_SRC ${SERVER_SRC}
 )
 endif(CURSES_FOUND)
 
-add_library(game ${ERESSEA_SRC})
+add_library(version OBJECT ${VERSION_SRC})
+add_library(game ${ERESSEA_SRC}
+  $<TARGET_OBJECTS:version>
+)
 add_executable(eressea ${SERVER_SRC})
 target_link_libraries(eressea
   game

src/kernel/CMakeLists.txt

@@ -33,7 +33,6 @@ jsonconf.test.c
 )
 
 SET(_FILES
-version.c
 alliance.c
 ally.c
 build.c
@@ -65,6 +64,8 @@ xmlreader.c
 jsonconf.c
 )
 
+SET(VERSION_SRC ${PROJECT_NAME}/version.c PARENT_SCOPE)
+
 FOREACH(_FILE ${_FILES})
     LIST(APPEND _SOURCES ${PROJECT_NAME}/${_FILE})
 ENDFOREACH(_FILE)