patch 8.1.0215: no error if configure --with-x cannot configure X

Problem:    No error if configure --with-x cannot configure X.
Solution:   Check that when --with-x is used X can be configured.
diff --git a/src/auto/configure b/src/auto/configure
index 1628b94..de1fd60 100755
--- a/src/auto/configure
+++ b/src/auto/configure
@@ -4466,6 +4466,8 @@
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $fail_if_missing" >&5
 $as_echo "$fail_if_missing" >&6; }
 
+with_x_arg="$with_x"
+
 if test -z "$CFLAGS"; then
   CFLAGS="-O"
   test "$GCC" = yes && CFLAGS="-O2 -fno-strength-reduce -Wall"
@@ -9003,6 +9005,10 @@
   fi
 fi
 
+if test "x$with_x" = xno -a "x$with_x_arg" = xyes; then
+    as_fn_error $? "could not configure X" "$LINENO" 5
+fi
+
 test "x$with_x" = xno -a "x$MACOS_X" != "xyes" -a "x$QNX" != "xyes" && enable_gui=no
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking --enable-gui argument" >&5