guile-snarf writes to $srcdir, which is not kosher

[Marius Vollmer]

Sigh, I've seen this coming...

guile-snarf writes its $cleanfile into the $srcdir, which is not a
supported thing to do during a VPATH build.

(This was uncovered by a simple "make distcheck", which thankfully
write-protects the source directory.  I hope everybody uses "make
distcheck" freely before checking in their changes, eh?)

It could either write the files into the build dir (which would be
difficult to do, when we want to support snarfing files that are not
in the current $srcdir), or it could stop trying to clean the input
altogether.

I'm in favor of the latter option since the cleaning is too much of a
kluge in my view.  It does appear to do the right thing, but only in a
situation where it is not needed: when someone snarfes directly to a
file (using "-o foo.x").  But in this case, the output file will
already exist, with a safe content, when the pre-processor is run
(modulo some theoretical buffering issues that are not going to
appear).  People who do not use "-o" always needed and still need to
protect against non existing .x files.  They can do that with the
(now) supported SCM_MAGIC_SNARFER macro.

This is bug "guile-snarf-writes-to-srcdir".