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>