- move using optimization, symbols (i.e. debug) and environment
    cflags/cxxflags into one place
- --enable-dbgutils is independent from --enable-debug, and thus
    also --enable-symbols, so it should not set debuglevel
- setting -g flag is controlled by --enable-symbols, not --enable-debug,
    so it should be used also for selective -g enabling
- setting debug flags depending on debuglevel being 2 is certainly wrong
- do not let environment cflags/cxxflags disable optimization/symbols flags
  completely, if one wants, it's possible to explicitly specify e.g. -g0,
  but the current way does not make it easily possible to specify e.g.
  -fcolor-diagnostics
- do not set debug flag globally now that it can be done selectively