Explicitly use OSX

Previously this was inherited via PBD.

On MacOS/X,  this adds
  "-undefined dynamic_lookup -flat_namespace"
and various "-framework .." options to linkflags

Without this flag, .dylibs fail to link usually because
of missing `-lintl` (Undefined symbols: "_libintl_dgettext")

On other systems this is a NO-OP:
CFLAGS_OSX, CXXFLAGS_OSX and LINKFLAGS_OSX
are only set on the darwin platform.

libs/backends/dummy/wscript

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