c++ Makefile - prevent overriding of CXXFLAGS and LDFLAGS

- the changes to the respective flags should be made with append
  operation. This prevents breaking/removing any default flags
  supplied by an external build system (e.g. distribution specifici
  package build system)
Merge pull request #7 from braiins/master

c++ Makefile - prevent overriding of CXXFLAGS and LDFLAGS