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/configure.ac b/src/configure.ac
index 107c170..0d8ea32 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -72,6 +72,9 @@
 	[fail_if_missing="no"])
 AC_MSG_RESULT($fail_if_missing)
 
+dnl Keep original value to check later.
+with_x_arg="$with_x"
+
 dnl Set default value for CFLAGS if none is defined or it's empty
 if test -z "$CFLAGS"; then
   CFLAGS="-O"
@@ -2283,6 +2286,11 @@
   fi
 fi
 
+dnl Check if --with-x was given but it doesn't work.
+if test "x$with_x" = xno -a "x$with_x_arg" = xyes; then
+    AC_MSG_ERROR([could not configure X])
+fi
+
 test "x$with_x" = xno -a "x$MACOS_X" != "xyes" -a "x$QNX" != "xyes" && enable_gui=no
 
 AC_MSG_CHECKING(--enable-gui argument)