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;
 }

src/configure.ac

@@ -3593,7 +3593,15 @@ fi
 
 AC_MSG_CHECKING(for SVR4 ptys)
 if test -c /dev/ptmx ; then
-  AC_TRY_LINK([], [ptsname(0);grantpt(0);unlockpt(0);],
+  AC_TRY_LINK([
+// These should be in stdlib.h, but it depends on _XOPEN_SOURCE.
+char *ptsname(int);
+int unlockpt(int);
+int grantpt(int);
+	       ], [
+		ptsname(0);
+		grantpt(0);
+		unlockpt(0);],
 	AC_MSG_RESULT(yes); AC_DEFINE(HAVE_SVR4_PTYS),
 	AC_MSG_RESULT(no))
 else

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2056,
 /**/
     2055,
 /**/