Avoid implicit function declarations in the configure tests, to
prevent build failures with future compilers.

Submitted upstream: <https://github.com/heimdal/heimdal/pull/1085>

diff --git a/cf/find-func-no-libs2.m4 b/cf/find-func-no-libs2.m4
index 5e5ed0e69ba60f7b..a6b3ad6d347adc94 100644
--- a/cf/find-func-no-libs2.m4
+++ b/cf/find-func-no-libs2.m4
@@ -21,7 +21,7 @@ if eval "test \"\$ac_cv_func_$1\" != yes" ; then
 		*) ac_lib="-l$ac_lib" ;;
 		esac
 		LIBS="$6 $ac_lib $5 $ac_save_LIBS"
-		AC_LINK_IFELSE([AC_LANG_PROGRAM([[$3]],[[$1($4)]])],[eval "if test -n \"$ac_lib\";then ac_cv_funclib_$1=$ac_lib; else ac_cv_funclib_$1=yes; fi";break])
+		AC_LINK_IFELSE([AC_LANG_PROGRAM([[char $1 (void);]],[[$1()]])],[eval "if test -n \"$ac_lib\";then ac_cv_funclib_$1=$ac_lib; else ac_cv_funclib_$1=yes; fi";break])
 	done
 	eval "ac_cv_funclib_$1=\${ac_cv_funclib_$1-no}"
 	LIBS="$ac_save_LIBS"
diff --git a/cf/have-struct-field.m4 b/cf/have-struct-field.m4
index bb7bcefbcc68a08c..3962d850645f88e4 100644
--- a/cf/have-struct-field.m4
+++ b/cf/have-struct-field.m4
@@ -7,7 +7,8 @@ dnl AC_HAVE_STRUCT_FIELD(struct, field, headers)
 AC_DEFUN([AC_HAVE_STRUCT_FIELD], [
 define(cache_val, translit(ac_cv_type_$1_$2, [A-Z ], [a-z_]))
 AC_CACHE_CHECK([for $2 in $1], cache_val,[
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[$3]],
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <string.h>
+$3]],
 	[[$1 x; memset(&x, 0, sizeof(x)); x.$2]])],
 	[cache_val=yes],
 	[cache_val=no])