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)