configure: test for functional readline more carefully

configure: test for functional readline more carefully

Apparently on (cirrus) macos, just testing for the ability to compile
readline.h isn't sufficient because configure ends up selecting the
built-in readline (which is insufficient) rather than the one we
specifically installed via brew.  More specifically, the built-in
readline has the wrong prototype for rl_completion_entry_function
which causes this error:

  _helpers.c:2096:38: error: incompatible function pointer types assigning to 'Function *' (aka 'int (*)(const char *, int)') from 'char *(const char *, int)' [-Werror,-Wincompatible-function-pointer-types]
        rl_completion_entry_function = on_completion_entry;
                                     ^ ~~~~~~~~~~~~~~~~~~~~

So change the test for an acceptable readline to check that
specifically, which is a better test for all the platforms.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>