conditional compilation

Hi,

Just a small suggestion - for picforth.fs to make gforth's conditional 
compilation words available, via:

     import: [if]
     import: [else]
     import: [then]
     import: [ifdef]
     import: [ifundef]
     ...

I've got these in my picforth site file, and I couldn't live without them.

-- 
Cheers
David