patch 8.2.2056: configure fails when building with implicit-function-declaration

Problem:    Configure fails when building with the
            "implicit-function-declaration" error enabled, specifically on Mac.
Solution:   Declear the functions like in the source code. (suggestion by
            Clemens Lang, closes #7380)

src/auto/configure

@@ -12350,10 +12350,18 @@ if test -c /dev/ptmx ; then
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
+// These should be in stdlib.h, but it depends on _XOPEN_SOURCE.
+char *ptsname(int);
+int unlockpt(int);
+int grantpt(int);
+
 int
 main ()
 {
-ptsname(0);grantpt(0);unlockpt(0);
+
+		ptsname(0);
+		grantpt(0);
+		unlockpt(0);
   ;
   return 0;
 }