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")