Fix some build and test failures with Autoconf 2.70.

Fix some build and test failures with Autoconf 2.70.

Autoconf 2.70 (released last week) makes a few changes that broke Automake’s
expectations, mostly in the test suite.  This patch addresses two of the
problems:

 - autoconf now issues a warning if fed a configure script that doesn’t invoke
   both AC_INIT and AC_OUTPUT; this, plus a problem with system-provided
   tools (still under investigation) broke the *build* on macOS; it also
   causes a couple of spurious testsuite failures.

 - AC_PACKAGE_NAME and AC_PACKAGE_VERSION are now defined unconditionally.
   AM_INIT_AUTOMAKE needs to use m4_ifset instead of m4_ifdef to diagnose
   the obsolete use of AC_INIT with fewer than two arguments.  (This change
   is compatible with autoconf 2.69; m4_ifset is much older, and it means
   ‘defined with a non-empty value’.)

* configure.ac: Use both AC_INIT and AC_OUTPUT in test configure scripts.
* t/deprecated-acinit.sh, t/init.sh: Likewise.
* m4/init.m4 (AM_INIT_AUTOMAKE): Use m4_ifset, not m4_ifdef, to detect
  AC_PACKAGE_NAME and/or AC_PACKAGE_VERSION not having a value.