Revert to the canonical way of customizing CXXFLAGS

A default list of compiler flags is set via AM_CXXFLAGS Automake variable.
However this gets overridden by per-target CXXFLAGS variables (e.g.
foo_CXXFLAGS in case the foo target). Because of this you should append
$(AM_CXXFLAGS) to the end of the per-target CXXFLAGS variables (e.g.
foo_CXXFLAGS = ... $(AM_CXXFLAGS)).

After this default list of flags the contents of the CXXFLAGS user variable is
passed to the compiler. This variable has a default value determined by
configure (in case of g++ it is '-g -O2'). You can override this by specifying
CXXFLAGS when invoking make (e.g. make CXXFLAGS='-O3').

 INCLUDE_DIRECTORIES(${CMAKE_SOURCE_DIR})

 LINK_DIRECTORIES(${CMAKE_BINARY_DIR}/lemon)

 SET(TESTS

   bfs_test

   counter_test

   dfs_test

   digraph_test

   dijkstra_test

   dim_test

   error_test

   graph_copy_test

   graph_test

   graph_utils_test

   heap_test

   kruskal_test

   maps_test

   max_matching_test

   random_test

   path_test

   time_measure_test

   unionfind_test)

 FOREACH(TEST_NAME ${TESTS})

   ADD_EXECUTABLE(${TEST_NAME} ${TEST_NAME}.cc)

   TARGET_LINK_LIBRARIES(${TEST_NAME} lemon)

   ADD_TEST(${TEST_NAME} ${TEST_NAME})

 ENDFOREACH(TEST_NAME)