Basically, this commit only simplifies configure.ac and makes the

mechanism more flexible, but otherwise should not affect your build
even if you regenerate the "configure" script.
(Most likely you need to run bootstrap.sh as you really need to re-run
autoheader for reasons that i do not completely understand).

If you don't regenerate "configure", of course you will see no difference.

In detail:

- restructure the check for mandatory modules to remove some
  redundant code blocks;

- extend the AST_EXT_LIB_CHECK so that it can used also for
  checking headers;

- define the AST_C_DEFINE_CHECK macro to test for #defined symbols;

- for the two above macros, add a last argument that getscopied into
  HAVE_$1_VERSION so the source can adapt to different versions of the
  same libraries/header/etc

- document the above;

- document a problem that existed before and i did not manage to solve:
  the 'description' argument to AC_DEFINE does not substiture shell variables
  so you will not see the actual values in the comments (in autoconfig.h)..




git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@44467 65c4cc65-6c06-0410-ace0-fbb531ad65f3