Fix PAM const-qualifier detection for Hammerhead/illumos.

xdm keys the `XDM_PAM_QUAL` const qualifier (used on the pam_conv callback
and pam_get_item() arguments) on `#ifdef __sun`, assuming any __sun target
uses the classic Solaris non-const PAM prototypes. Hammerhead's GCC defines
__sun, but its <security/pam_appl.h> defaults to the CONST-qualified form
(int (*conv)(int, const struct pam_message **, ...) and pam_get_item(...,
const void **)) unless _PAM_LEGACY_NONCONST is defined. With __sun alone,
XDM_PAM_QUAL expands empty (non-const) and greet.c fails to compile under
GCC 14 with -Wincompatible-pointer-types (now a hard error):

  greet.c:496: initialization of 'int (*)(int, const struct pam_message **,...)'
               from incompatible pointer type 'int (*)(int, struct pam_message **,...)'
  greet.c:588/613/735: passing argument 3 of 'pam_get_item' from incompatible
               pointer type

Gate the non-const branch on _PAM_LEGACY_NONCONST as well, so the qualifier
matches whichever prototype form the PAM headers actually present. On
Hammerhead this selects `const`, matching the default headers; on a legacy
Solaris that defines _PAM_LEGACY_NONCONST it stays empty as before.

--- a/greeter/greet.c
+++ b/greeter/greet.c
@@ -152,9 +152,13 @@ static XtIntervalId	pingTimeout;

 #ifdef USE_PAM

-#ifdef __sun
-/* Solaris does not const qualify arguments to pam_get_item() or the
-   PAM conversation function that Linux-PAM and others do. */
+#if defined(__sun) && defined(_PAM_LEGACY_NONCONST)
+/* Classic Solaris did not const qualify arguments to pam_get_item() or the
+   PAM conversation function that Linux-PAM and others do. Hammerhead's
+   <security/pam_appl.h> defaults to the const-qualified (non-legacy) form
+   unless _PAM_LEGACY_NONCONST is defined, and Hammerhead's GCC also defines
+   __sun, so key the qualifier on _PAM_LEGACY_NONCONST rather than on __sun
+   alone. */
 # define XDM_PAM_QUAL /**/
 #else
 # define XDM_PAM_QUAL const