compat: reinstate AM_CONFIG_HEADER and AM_PROG_CC_STDC

compat: reinstate AM_CONFIG_HEADER and AM_PROG_CC_STDC

Make them give runtime warnings in the obsolete category, but apart
from that, make them behave as they did in Automake 1.12.x and earlier.

While removing those macros seemed quite harmless, because it didn't put
a real burden on the developers (requiring them just to do a quick edit
to configure.ac), it turned out to place an unsustainable burden (or at
least, a burden perceived as such) on distro packagers who use the latest
Automake to bootstrap existing packages.  Many of those packages, while
having likely updated to AC_CONFIG_HEADERS in their development version,
still used AM_CONFIG_HEADER in their existing released versions, and the
removal of this macro would have thus forced the Fedora packagers to
patch all of them.  References:

  <http://www.mail-archive.com/devel@lists.fedoraproject.org/msg52840.html>
  <http://www.spinics.net/lists/fedora-devel/msg175922.html>
  <http://blog.flameeyes.eu/2013/01/autotools-mythbuster-automake-pains>

In addition, the Fedora packagers have already decided to patch their
Automake 1.13.1 to reinstate the AM_CONFIG_HEADER and AM_PROG_CC_STDC
macros (plus other macros that I don't believe it's worth worrying about):

  <http://www.spinics.net/lists/fedora-devel/msg176098.html>
  <http://www.mail-archive.com/devel@lists.fedoraproject.org/msg53030.html>
  <http://pkgs.fedoraproject.org/cgit/automake.git/commit/?id=ffe6bc39>

So, rather than having one more incompatibility floating around, we
better mirror that change (or, actually, its relevant parts) in the
upstream.

* m4/obsolete-err.m4 (AM_CONFIG_HEADER, AM_PROG_CC_STDC): Revert to the
older semantics, plus a runtime warning in the 'obsolete' category.
* t/backcompat6.sh: Use AM_CONFIG_HEADER once again.
* t/am-config-header-no-more.sh: Rename ...
* t/am-config-header.sh: ... like this, and adjust.
* t/am-prog-cc-stdc-no-more.sh: Rename ...
* t/am-prog-cc-stdc.sh: ... like this, and adjust.
* t/list-of-tests.mk: Adjust.
* NEWS: Update.

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