doc: Don't leave a broken .texi file around

[Bruno Haible]

Hi,

When building with CFLAGS="-O0 -ggdb", I got a non-functional 'poke' executable,
and, as a result, the file doc/poke-settings.texi was created, as a 1-line file
with the contents

  FATAL ERROR: buffer overflow in writing executable code: crash now, in case 
we have not crashed already

Even after fixing the mistake, that broken .texi file was still there.

This proposed patch fixes it, by touching the .texi file only when the command
succeeded.

0001-doc-Don-t-leave-a-broken-.texi-file-around.patch
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