Reduce warnings from mainline autoconf

Reduce warnings from mainline autoconf

In configure.ac and aclocal.m4, eliminate the use of old macros that
will generate warning from the forthcoming autoconf 2.70 release.
Specifically:

* Use AS_HELP_STRING instead of AC_HELP_STRING.

* Use AC_{COMPILE,LINK,RUN}_IFELSE and AC_LANG_{SOURCE,PROGRAM}
instead of AC_TRY_COMPILE and similar.

* Use m4_foreach_w instead of ac_foreach.

* Eliminate AC_PROG_LEX and yylineno checking, as we no longer use
  lex.

* As recommended by autoconf, assume that signal handlers return void
  as specified in C89.

* As recommmended by autoconf, assume <time.h> is present and that
  <sys/time.h>, if present, can be included alongside it.

* Don't call AC_CHECK_FUNCS with a shell variable for the thread
  safety checks.  Instead just assume (as is currently the case) that
  all of the functions have been previously checked.