Fix test failure of autopoint-3 on Cygwin.

Fix test failure of autopoint-3 on Cygwin.

It was broken since commit "tests: Use libtool in autopoint-3 test" (2014-09-05):
The Makefile specified to link the 'hello' program without libtool
(and thus Automake generated a direct link command without libtool),
but the LDADD variable used @LTLIBINTL@. On Cygwin, this led to a use of
the -R option, which the compiler does not understand:
x86_64-pc-cygwin-gcc: error: unrecognized command line option ‘-R’; did you mean ‘-R’?

* gettext-tools/tests/autopoint-3: Don't invoke LT_INIT. Use @LIBINTL@,
not @LTLIBINTL@. Don't touch ltmain.sh.