conditional prerequisite

[Thomas . Wolff]

Hello,
How can I achieve the following?
Suppose, I have a dependency that extracts compilable information 
from a data source, and another rule that retrieves the data source 
e.g. from the web:

info.h: info.txt
        sed -f info.sed info.txt > info.h

info.txt:
        wget http://blabla/info.txt

Now of course I want info.h to be made if info.txt exists and 
is newer.
But it is also possible that info.txt is not available but info.h 
is still there. In this case I want it to be done and not try to 
retrieve info.txt (so I might actually leave out the second rule).

How can this be done?

Thanks and kind regards,
Thomas Wolff