Remove flag.cc from weyland.gyp.

The Flag class is now implemented entirely by the flag.h header
file.
We can now parse bool flags, sorta.

We can now handle the case where users provide a bool flag but
don't specify the value of the flag. We also don't handle the "-"
or "/" character for them. They have to include those characters
in the flag registration.

So really we don't support this feature very well at all. But it's
a start.