AC_CHECK_LIB: retest after failure

[Dave Milter]

Hi!

Using autoconf I try to implement such scenario:

check if compilation with libraryr foo and function bar succeeded,
if no define variable DUMMY and retest again,
so I try to do things in this way:

AC_CHECK_LIB(mytest, bar, fooLIBS=-lfoo, $CONFLIB_ARG)#(1)
if test -z "$fooLIBS"; then#(2)
  AC_DEFINE_UNQUOTED(DUMMY, DDD)#(3)
  AC_CHECK_LIB(mytest, bar, fooLIBS=-lfoo, $CONFLIB_ARG)#(4)
fi#(5)

on system where (1) working, all fine, but on system where (1) not working,
(4) also failes, because of compiler is not executed,
if comment (1) all works fine on systems where (1) failed,

so my question, how can I remove cache entries which
create line (1), or how to force (4) even if cache entries says that (1) failed?