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/widgets/wscript

@@ -75,7 +75,7 @@ def build(bld):
 
     obj.export_includes = ['.']
     obj.includes = ['.'] + bld.env['INCLUDES_GLIB']
-    obj.uselib       = 'SIGCPP CAIROMM GTKMM BOOST XML'
+    obj.uselib       = 'SIGCPP CAIROMM GTKMM BOOST XML OSX'
     obj.use          = [ 'libpbd', 'libgtkmm2ext' ]
     obj.name         = 'libwidgets'
     obj.target       = 'widgets'