configure.ac, fix `NO_X' and `has_no_help'

drop configure options `--enable-x' and `--disable-x'
use `--with-x' and `--without-x' provided by AC_PATH_X

bug tracker item:
  [ngspice-Bugs-3515015] Startup error message without x11
  https://sourceforge.net/tracker/index.php?func=detail&aid=3515015&group_id=38962&atid=423915

configure.ac

@@ -167,10 +167,6 @@ dnl Xgraph is a plotting package for X11 once very popular.
 AC_ARG_ENABLE(xgraph,
     AS_HELP_STRING([--enable-xgraph],[Enable xgraph compilation.]))
 
-dnl --enable-x: Compile software using x libraries. A GUI will be provided then.
-AC_ARG_ENABLE(x,
-    AS_HELP_STRING([--enable-x],[Enable X11 gui]))
-
 dnl --with-readline:  Includes GNU readline support into CLI.  Default is "no".
 AC_ARG_WITH(readline,
     AS_HELP_STRING([--with-readline[=yes/no]],[Enable GNU readline support for CLI. Default=no.]))
@@ -490,7 +486,7 @@ case $with_windows in
 		if test "x$enable_help" = "xyes"; then
 		  has_no_help=false
 		else
-		  Has_no_help=true
+		  has_no_help=true
 		fi
 		CFLAGS="$CFLAGS -mwindows";;
 	* )
@@ -502,7 +498,10 @@ dnl Checks for X11 header files and libraries - X11 support can be disabled
 dnl by passing the '--without-x' option to configure:
 
 dnl Try to locate the X-Windows include files and libraries
+dnl   will set no_x="yes" or no_x=""
 AC_PATH_X
+
+dnl will set X_CFLAGS, X_LIBS, and might define X_DISPLAY_MISSING
 AC_PATH_XTRA
 
 # Checks for typedefs, structures, and compiler characteristics.
@@ -586,10 +585,8 @@ dnl for MacOSX or Linux, but
 dnl			-lXaw -lXmu -lXt -lXext -lX11
 dnl seems to be the popular choice.
 dnl (The previous order was -lX11 -lXt -lXext -lXmu -lXaw)
-if test "x$enable_x" = "xno"; then
-	no_x = "no"
-fi
-if test ! "$no_x" = "yes" ; then
+
+if test "x$no_x" != "xyes" ; then
 	AC_CHECK_LIB(Xaw,main,X_LIBS="$X_LIBS -lXaw",AC_MSG_ERROR(Couldn't find Xaw library),$X_LIBS  $X_EXTRA_LIBS)
 	AC_CHECK_LIB(Xmu,main,X_LIBS="$X_LIBS -lXmu",AC_MSG_ERROR(Couldn't find Xmu library), $X_LIBS $X_EXTRA_LIBS)
 	X_LIBS="$X_LIBS -lXt"

src/frontend/display.c

@@ -171,7 +171,7 @@ DevInit(void)
 /* console application under MS Windows */
 #if !defined(HAS_WINDOWS) && !defined(TCL_MODULE) && (defined(_MSC_VER) || defined(__MINGW32__))
         fprintf(cp_err, "Warning: no graphics interface!\n You may use command 'gnuplot'\n if GnuPlot is installed.\n");
-#else
+#elif !defined(X_DISPLAY_MISSING)
         externalerror(
 	         "no graphics interface;\n please check if X-server is running,\n or ngspice is compiled properly (see INSTALL)");
 #endif