libstdc++: Reduce output of 'make doc-pdf-doxygen'

libstdc++: Reduce output of 'make doc-pdf-doxygen'

Use '@' to prevent Make from echoing the recipe, so that users don't see
this every time:

  if [ -f ${doxygen_pdf} ]; then
    mv ${doxygen_pdf} ${api_pdf} ;
    echo ":: PDF file is ${api_pdf}";
  else
    echo "... error";
    grep -F 'LaTeX Error' ${doxygen_outdir}/latex/refman.log;
    grep -F 'TeX capacity exceeded, sorry' ${doxygen_outdir}/latex/refman.log;
    exit 12;
  fi

The presence of the "error" strings in the output makes it look like an
error happened. By suppressing the echoing user's will only see "error"
if the 'else' branch is taken.

libstdc++-v3/ChangeLog:

	* doc/Makefile.am (stamp-pdf-doxygen): Improve comment about
	dealing with errors. Use '@' to prevent shell command being
	echoed.
	* doc/Makefile.in: Regenerate.