This modifies the behavior of the '-e' flag to PCbuild\build.bat: when '-e'
is not supplied, no attempt will be made to build extension modules that
require external libraries, even if the external libraries are present.

Also adds '--no-<module>' flags to PCbuild\build.bat, where '<module>' is
one of 'ssl', 'tkinter', or 'bsddb', to allow skipping just those modules
(if '-e' is given).