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

gcc/ChangeLog

+2018-08-10  Thomas Preud'homme  <thomas.preudhomme@linaro.org>
+
+	* Makefile.in: Clarify which tm.texi to copy over to assert the
+	right to grant a GFDL license for all.
+
 2018-08-09  Jeff Law  <law@redhat.com>
 
 	* config/m68k/m68k.c (m68k_adjust_decorated_operand): Remove

gcc/Makefile.in

@@ -2504,7 +2504,7 @@ s-tm-texi: build/genhooks$(build_exeext) $(srcdir)/doc/tm.texi.in
 	else \
 	  echo >&2 ; \
 	  echo Verify that you have permission to grant a GFDL license for all >&2 ; \
-	  echo new text in tm.texi, then copy it to $(srcdir)/doc/tm.texi. >&2 ; \
+	  echo new text in $(objdir)/tm.texi, then copy it to $(srcdir)/doc/tm.texi. >&2 ; \
 	  false; \
 	fi