Add —disable-gui option

Prevents showing a GUI message box if the user wants to run from the
console.

As requested here:
   https://sourceforge.net/p/flightgear/codetickets/2306/

src/GUI/MessageBox.cxx

@@ -43,6 +43,8 @@ using namespace simgear::strutils;
 
 namespace {
 
+static bool static_isHeadless = false;
+
 bool isCanvasImplementationRegistered()
 {
 	if (!globals) {
@@ -107,12 +109,22 @@ win32MessageBox(const std::string& caption,
 namespace flightgear
 {
 
+void setHeadlessMode(bool headless)
+{
+    static_isHeadless = headless;
+}
+
+bool isHeadlessMode()
+{
+    return static_isHeadless;
+}
+
 MessageBoxResult modalMessageBox(const std::string& caption,
     const std::string& msg,
     const std::string& moreText)
 {
     // Headless mode.
-    if (globals->is_headless()) {
+    if (static_isHeadless) {
         SG_LOG(SG_HEADLESS, SG_ALERT, "ModalMessageBox Caption: \"" << caption << "\"");
         SG_LOG(SG_HEADLESS, SG_ALERT, "ModalMessageBox Message: \"" << msg << "\"");
         if (!moreText.empty())
@@ -161,6 +173,15 @@ MessageBoxResult fatalMessageBoxWithoutExit(const std::string& caption,
     const std::string& msg,
     const std::string& moreText)
 {
+    // Headless mode.
+    if (static_isHeadless) {
+        SG_LOG(SG_HEADLESS, SG_ALERT, "Fatal Error: \"" << caption << "\"");
+        SG_LOG(SG_HEADLESS, SG_ALERT, "Error Message: \"" << msg << "\"");
+        if (!moreText.empty())
+            SG_LOG(SG_HEADLESS, SG_ALERT, "\tMore text: \"" << moreText << "\"");
+        return MSG_BOX_OK;
+    }
+
 #if defined(SG_WINDOWS)
     return win32MessageBox(caption, msg, moreText);
 #elif defined(SG_MAC)

src/GUI/MessageBox.hxx

@@ -7,6 +7,10 @@
 namespace flightgear
 {
 
+// set a global value indicating we're in headless mode.
+void setHeadlessMode(bool headless);
+bool isHeadlessMode();
+
 // special exception class used to signal an exit. Must not inherit
 // std::exception or similar, since we want to handle it specially
 class FatalErrorException

src/Main/bootstrap.cxx

@@ -233,6 +233,12 @@ int _bootstrap_OSInit;
 // Main entry point; catch any exceptions that have made it this far.
 int main ( int argc, char **argv )
 {
+    // we don't want to accidently show a GUI box and block startup in
+    // non_GUI setups, so check this value early here, before options are
+    // processed
+    const bool headless = flightgear::Options::checkForArg(argc, argv, "disable-gui");
+    flightgear::setHeadlessMode(headless);
+
 #ifdef ENABLE_SIMD
   if (!detectSIMD()) {
     flightgear::fatalMessageBoxThenExit(

src/Main/globals.cxx

@@ -165,8 +165,7 @@ FGGlobals::FGGlobals() :
     channel_options_list( NULL ),
     initial_waypoints( NULL ),
     channellist( NULL ),
-    haveUserSettings(false),
+    haveUserSettings(false)
-    _headless(false)
 {
     SGPropertyNode* root = new SGPropertyNode;
     props = SGPropertyNode_ptr(root);
@@ -951,12 +950,12 @@ void FGGlobals::setPackageRoot(const SGSharedPtr<simgear::pkg::Root>& p)
 
 bool FGGlobals::is_headless()
 {
-    return _headless;
+    return flightgear::isHeadlessMode();
 }
 
 void FGGlobals::set_headless(bool mode)
 {
-    _headless = mode;
+    flightgear::setHeadlessMode(mode);
 }
 
 // end of globals.cxx

src/Main/globals.hxx

@@ -159,8 +159,6 @@ private:
 
     SGSharedPtr<simgear::pkg::Root> _packageRoot;
 
-    bool _headless;
-
 public:
 
     FGGlobals();

src/Main/options.cxx

@@ -1600,7 +1600,11 @@ fgOptLoadTape(const char* arg)
   return FG_OPTIONS_OK;
 }
 
-
+static int fgOptDisableGUI(const char*)
+{
+    globals->set_headless(true);
+    return FG_OPTIONS_OK;
+}
 
 /*
    option       has_param type        property         b_param s_param  func
@@ -1865,6 +1869,7 @@ struct OptionDesc {
     {"load-tape",                    true,  OPTION_FUNC,   "", false, "", fgOptLoadTape },
     {"developer",                    true,  OPTION_IGNORE | OPTION_BOOL, "", false, "", nullptr },
     {"jsbsim-output-directive-file", true,  OPTION_STRING, "/sim/jsbsim/output-directive-file", false, "", nullptr },
+    {"disable-gui",                  false, OPTION_FUNC, "", false, "", fgOptDisableGUI },
     {nullptr,                        false, 0,             nullptr, false, nullptr, nullptr}
 };