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})

ADD_LIBRARY(lemon

  arg_parser.cc

  base.cc

  color.cc

  random.cc)

INSTALL(

  TARGETS lemon

  ARCHIVE DESTINATION lib

  COMPONENT library)

INSTALL(

  DIRECTORY . bits concepts

  DESTINATION include/lemon

  COMPONENT headers

  FILES_MATCHING PATTERN "*.h")