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/surfaces/control_protocol/wscript

@@ -33,7 +33,7 @@ def build(bld):
     obj.name         = 'libardour_cp'
     obj.target       = 'ardourcp'
     obj.use          = 'libardour libpbd'
-    obj.uselib       = 'GLIBMM SIGCPP XML'
+    obj.uselib       = 'GLIBMM SIGCPP XML OSX'
     obj.install_path = bld.env['LIBDIR']
 
 def shutdown():