Teach the configure script to look for an ar tool prefixed with the
target triple (i.e., `<target>-ar`) when cross compiling. This matches
the behavior for tools that have built-in autoconf macros, like ranlib.
(For some reason there is no AC_PROG_AR macro.)
Also, remove the AC_PROG_ARCHIVE and AC_PROG_ARCHIVE_ADD macros, which
have been dead code since 780b34cd.
With this change, cross compiling libkrb5 works a bit better out of the