Avoid implicit function declarations, so that the configure probe
results do not change with future compilers.

Submitted upstream for discussion:

  <https://www.sudo.ws/pipermail/sudo-workers/2023-April/001387.html>


diff --git a/configure b/configure
index d406eb77a22d3c3c..29483788443d2b21 100755
--- a/configure
+++ b/configure
@@ -31220,10 +31220,13 @@ else case e in #(
 /* end confdefs.h.  */
 #include <sys/types.h>
 #include <ldap.h>
+
+void *volatile ptr;
+
 int
 main (void)
 {
-(void)ldap_init(0, 0)
+ptr = (void *) ldap_msgfree
   ;
   return 0;
 }
@@ -33914,7 +33917,7 @@ then :
 else case e in #(
   e) cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-
+#include <stdio.h>
 
 int
 main (void)
diff --git a/m4/hardening.m4 b/m4/hardening.m4
index f7d2a8c2911ed9d6..1ebfd9fdaf461285 100644
--- a/m4/hardening.m4
+++ b/m4/hardening.m4
@@ -10,7 +10,7 @@ AC_DEFUN([SUDO_CHECK_HARDENING], [
 	    [sudo_cv_use_fortify_source],
 	    [AC_LINK_IFELSE([
 		    AC_LANG_PROGRAM(
-			[[]], [[char buf[4]; (void)sprintf(buf, "%s", "foo");]]
+			[[#include <string.h>]], [[char buf[4]; (void)sprintf(buf, "%s", "foo");]]
 		    )],
 		    [sudo_cv_use_fortify_source=yes],
 		    [sudo_cv_use_fortify_source=no]
diff --git a/m4/ldap.m4 b/m4/ldap.m4
index 78c21e0bc0a1f65f..a6361df044d84f92 100644
--- a/m4/ldap.m4
+++ b/m4/ldap.m4
@@ -52,7 +52,10 @@ AC_DEFUN([SUDO_CHECK_LDAP], [
 #include <lber.h>])
 	AC_CACHE_CHECK([whether lber.h is needed when including ldap.h], [sudo_cv_header_lber_h], [
 	    AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include <sys/types.h>
-#include <ldap.h>]], [[(void)ldap_init(0, 0)]])], [
+#include <ldap.h>
+
+void *volatile ptr;
+]], [[ptr = (void *) ldap_msgfree]])], [
 		# No need to explicitly include lber.h when including ldap.h.
 		sudo_cv_header_lber_h=no
 	    ], [