Fix linking of C++ unit tests with CMake

The problem with them is that the "libraries" into which Wesnoth is split
have circular dependencies, and linking of the game succeeds only by chance
(everything just happens to be used by the time the linker encounters it).

Fixed by allowing the linker to use multiple passes over the libraries
until all references are resolved.

src/CMakeLists.txt

@@ -277,6 +277,7 @@ if(ENABLE_GAME)
 	add_executable(wesnoth WIN32 wesnoth.cpp)
 
 	target_link_libraries(wesnoth
+		-Wl,--start-group
 		wesnoth-client
 		lua
 		wesnoth-core
@@ -284,6 +285,7 @@ if(ENABLE_GAME)
 		wesnoth-sdl
 		${WIDGETS_LIB}
 		wesnoth-client
+		-Wl,--end-group
 		${game-external-libs}
 	)
 	set_target_properties(wesnoth
@@ -357,6 +359,7 @@ if(ENABLE_TESTS)
 	add_executable(boost_unit_tests ${test_SRC})
 
 	target_link_libraries(boost_unit_tests
+		-Wl,--start-group
 		wesnoth-client
 		lua
 		wesnoth-core
@@ -364,6 +367,7 @@ if(ENABLE_TESTS)
 		wesnoth-sdl
 		${WIDGETS_LIB}
 		wesnoth-client
+		-Wl,--end-group
 		${game-external-libs}
 		boost_unit_test_framework
 	)