SConstruct: further refinement of debug control at build time. A new DEBUG_MESSAGES option controls whether debug messages are printed (default is enabled). DEBUG now turns on warnings, symbols and additional debug code (default is enabled). If DEBUG is not enabled a '-O2' build is done. The DEBUG_BUILD option added in r2507 has been removed. Production builds are encouraged to leave DEBUG_MESSAGES enabled while disabling DEBUG.
git-svn-id: svn+ssh://ffado.org/ffado/trunk@2517 2be59082-3212-0410-8809-b0798e1608f0