Fix opening links to in prefs notes (e.g. VST spec)

gtk's default URL open does not work on recent macOS and
Windows.

gtk2_ardour/option_editor.cc

@@ -42,6 +42,7 @@
 
 #include "pbd/configuration.h"
 #include "pbd/replace_all.h"
+#include "pbd/openuri.h"
 #include "pbd/strsplit.h"
 
 #include "widgets/frame.h"
@@ -110,6 +111,10 @@ OptionEditorComponent::maybe_add_note (OptionEditorPage* p, int n)
 		l->set_use_markup (true);
 		l->set_line_wrap (true);
 		p->table.attach (*l, 1, 3, n, n + 1, FILL | EXPAND);
+		if (_note.find ("<a href=") != _note.npos) {
+			l->property_track_visited_links() = false;
+			l->signal_activate_link().connect ([](std::string const& url) { return PBD::open_uri (url); });
+		}
 	}
 }