libltdl: edit AM_CPPFLAGS correctly for libltdl/Makefile.am.

* libltdl/ltdl.mk (AM_CPPFLAGS): Make sure the sed expression to
remove the first libltdl/ on each line is not confused by misuse
of linebreaks.
* Makefile.am (lt_Makefile_am): Also edit out the duplicated
include paths after libltdl/ elimination.
* NEWS: Update.
Reported by Michael Wobst

Signed-off-by: Gary V. Vaughan <gary@gnu.org>