m4: readline: Comment rl_completion_quote_character() check

The check was added in 74416b1d4849 without offering any
explanation outside of the commit message. Introduce a comment
to make digging through the git history unnecessary.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

diff --git a/m4/virt-readline.m4 b/m4/virt-readline.m4
index 4823a27788a0f0f17a061d872ec8f3b7b429bed0..649a52edfa3ffee1f6f120244bfe9d82336ebebc 100644 (file)
--- a/m4/virt-readline.m4
+++ b/m4/virt-readline.m4
@@ -37,6 +37,12 @@ AC_DEFUN([LIBVIRT_CHECK_READLINE],[
     LIBS="$lv_saved_libs $extra_LIBS"
   fi
 
+  # This function is present in all reasonable (5.0+) readline versions;
+  # however, the macOS base system contains a library called libedit which
+  # takes over the readline name despite lacking many of its features. We
+  # want to make sure we only enable readline support when linking against
+  # the actual readline library, and the availability of this specific
+  # functions is as good a witness for that fact as any.
   AC_CHECK_DECLS([rl_completion_quote_character],
                  [], [],
                  [[#include <stdio.h>