Algol68: do not clobber A68FLAGS in AC_PROG_A68 (sr #111382)

The AC_PROG_A68 macro should honor the A68FLAGS set by the user at
configure time.

Fixes <https://savannah.gnu.org/support/?111382>.

* lib/autoconf/a68.m4: Do not clobber A68FLAGS in AC_PROG_A68.
  Simplify sequence of AC_CHECK_TOOL/AC_CHECK_PROGS operations;
  this macro does not have the backward compatibility constraints
  that AC_PROG_CC does.