[krbdev.mit.edu #1342] fix for requiring gawk

[Dave Love]

"Tom Yu via RT" <rt-comment at krbdev.mit.edu> writes:

> I think the thing to do would be to explicitly override the setting of
> the AWK variable on the make command line if you need to actually use
> the top-level targets that need it, e.g. kerbsrc.zip.

Indeed; I don't see why the normal case should lose.  Failure to build
on anything without gawk installed as such is rather unfortunate.