require: autoconf >= 2.65; related simplifications

require: autoconf >= 2.65; related simplifications

The new support for Objective C++ (to be added in Automake 1.12.1) can be
simplified if we assume Autoconf version 2.65 or later (2.65 being the
version that introduced support for Objective C++).  Since such an Autoconf
version is two and half years old now (and will likely be almost three
years old when Automake 1.13 gets released), requiring it is acceptable.
This will also simplify testing and maintenance of Automake, because we'll
need to test with fewer Autoconf version, and possibly rely on new Autoconf
features.

* NEWS (Version requirements): Automake 1.13 will require Autoconf
2.65 or later.
* configure.ac ($required_autoconf_version): Bump to 2.65.
* m4/init.m4 (AM_INIT_AUTOMAKE): Require Autoconf >= 2.65.
Assume AC_PROG_OBJCXX is unconditionally defined.
* m4/depout.m4: Adjust a comment about a nugget of defensive programming.
* t/ext.sh: Assume that AC_PROG_OBJCXX is unconditionally defined,
and that we are using Autoconf >= 2.65.  Related simplifications.
* t/objc-megademo.sh: Likewise.
* t/objcxx-basic.sh: Likewise.
* t/objcxx-deps.sh: Likewise.
* t/objcxx-flags.sh: Likewise.
* t/objcxx-minidemo.sh: Likewise.
* t/nodep2.sh: Likewise.
* t/backcompat3.sh: Assume AC_INIT accepts an URL argument.
* t/depend5.sh: Adjust a comment.
* syntax-checks.mk (sc_test_names): Delete this check as now useless:
autoconf >= 2.65 (>= 2.63, actually) can handle arguments to AC_INIT
that contain or are m4 builtins or predefined macros.
(m4_builtin): Delete this now-useless variable as well, was used only
by the check above.
(syntax_check_rules): Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>