build: don't hardcode repeated long paths in Makefile rules.

build: don't hardcode repeated long paths in Makefile rules.

According to the DRY principle, scattering several copies of
something across a file or project leads to pain.  Let's not
do that!
* Makefile.am (Makefile_in, Makefile_inc, defs_in, defs_m4sh),
(libtoolize_in, ltmain_m4sh, ltversion_in): New variables to
hold the location of important files and targets.
(EXTRA_DIST): Add the distributed ones by variable.
(libtoolize, libltdl/Makefile.am, libltdl/m4/ltversion.m4)
(libltdl/config/ltmain.sh, libltdl/stamp-mk, tests/defs)
(tests/defs.in): Use the new variables to reduce noise and
repetition.

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