libtool: suggest LT_INIT if LTLIBRARIES primary is used

libtool: suggest LT_INIT if LTLIBRARIES primary is used

When the LTLIBRARIES primary was used, but $(LIBTOOL) wasn't
defined, automake suggested to add a call to AC_PROG_LIBTOOL
in configure.ac.  But that macro is deprecated since Libtool
version 1.9b (2004-08-29), in favor of the newer LT_INIT.  So
suggest the use of this latter macro instead.

* lib/Automake/Variable.pm (%_am_macro_for_var): Pair 'LIBTOOL'
with 'LT_INIT', not with 'AC_PROG_LIBTOOL'.
* libtool4.test: Adjust and extend.  Also, add a call to macro
AC_PROG_CC in configure.in, to help ensuring that automake does
not fail for the wrong reasons.
* ltinit.test: New test, ensure that automake's libtool support
works with LT_INIT-based interface.

Thanks to Jack Kelly for the suggestion.