Fix Makefile export with Python 3

In Python 3, the map() function returns a map object, not a list object as in
Python 2. Ensure a list object is returned from format_flags() by wrapping
map() in list(). This is compatible with both Python 2 and 3.

tools/export/makefile/__init__.py

@@ -165,7 +165,7 @@ class Makefile(Exporter):
         flags = {}
         for k, v in self.flags.items():
             if k in ['c_flags', 'cxx_flags']:
-                flags[k] = map(lambda x: x.replace('"', '\\"'), v)
+                flags[k] = list(map(lambda x: x.replace('"', '\\"'), v))
             else:
                 flags[k] = v