Clarify source of tm.texi to copy for GFDL grant

Clarify source of tm.texi to copy for GFDL grant

When tm.texi.in is updated in the source tree, the following message
gets displayed:

Verify that you have permission to grant a GFDL license for all
new text in tm.texi, then copy it to <gcc src dir>/gcc/doc/tm.texi.

Having been myself and some colleagues confused several time by that
message as to what tm.texi to copy, I think it would be clearer to
indicate the absolute path for the source as well. This patch achieves
that.

2018-08-10  Thomas Preud'homme  <thomas.preudhomme@linaro.org>

    gcc/
    * Makefile.in: Clarify which tm.texi to copy over to assert the
    right to grant a GFDL license for all.

From-SVN: r263464