Remove unneeded checks from configure.ac.

HAVE_{BI,STD,FWD}_ITERATOR and HAVE_NAMESPACES were not used in the code.
bison and flex are no longer used.
CAN_DLOPEN_SELF was never used either.
AC_PROG_LIBTOOL is not needed since we don't use libtool, we only need the
libltdl checks for dlopen.
Add check for AR, it used to be done by AC_PROG_LIBTOOL.
AC_TYPE_SIGNAL is deprecated, follow autoupdate's suggestion and replace with
void.
Remove unused m4 files.
Configure can now be generated using autoconf 2.65 too, without any warnings!

llvm-svn: 94534