specifying pure space without editing puresize.h

[Stephen J. Turnbull]

Ivan Kanis writes:

 > Building Emacs overflowed pure space

It is knowable (and perhaps known to the build process already) how
much overflow there was.  XEmacs just builds the executable twice,
once with an approximate figure, and then with an exact one based on a
report from the first build.  Can't recall any problems with it, and
it's only a few extra seconds on a modern machine.