prepare for update to waf 1.8

uselib is no longer implicit (inherited by .use). This is still incomplete,
some uselibs for non-linux variants may be missing.

bld.is_defined("HAVE_XXX") also no longer works and will have to be
changed (I think to bld.env["HAVE_XXX"]) in countless places.

libs/backends/dummy/wscript

@@ -25,6 +25,7 @@ def build(bld):
     obj.name     = 'dummy_audiobackend'
     obj.target   = 'dummy_audiobackend'
     obj.use      = 'libardour libpbd'
+    obj.uselib   = 'GLIBMM XML'
     obj.install_path  = os.path.join(bld.env['LIBDIR'], 'backends')
     obj.defines = ['PACKAGE="' + I18N_PACKAGE + '"',
                    'ARDOURBACKEND_DLL_EXPORTS'