scons: the 'unused-but-set-variable' added in gcc 4.6 triggers warnings when compilin...

scons: the 'unused-but-set-variable' added in gcc 4.6 triggers warnings when compiling code generated by dbusxx-xml2cpp (such as controlserver-glue.h).  This is caused by numerous instances of the call.reader() return value being stored but not used.  Since we can do nothing about this, suppress this warning when compiling dbus-related files.  This change plus those in r2522 to r2526 inclusive allow FFADO to compile without warnings, at least under gcc 4.7.x.

git-svn-id: svn+ssh://ffado.org/ffado/trunk@2527 2be59082-3212-0410-8809-b0798e1608f0