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/fst/wscript

@@ -77,7 +77,7 @@ def build(bld):
             obj.linkflags = ['-mwindows']
             obj.use = [ 'libpbd', 'vstwin' ]
         else:
-            obj.uselib = ['GIOMM', 'DL']
+            obj.uselib = ['GIOMM', 'DL', 'OSX']
             obj.use = [ 'libpbd' ]
 
     obj.includes  = [ '../pbd/', '../ardour/', '.' ]